Some somewhat random thoughts on this issue. You could write a book about this, but here are some important considerations
Very hard thing to choose a martial art/self-defense style but the first thing I would ask is "what does this style envisage as constituting a fight/street fight?" One on one? Multiple opponents? Spontaneous explosion or "let's go outside and settle this"?
And what techniques are "allowed"? This second is important because how you train will be pretty much how you fight. If you have trained only with closed fists, then you will never, for example, slap a guy on the top or back of his head with an open hand yet this hand and this target are very often available in fighting.
Once you have that statement, consider whether they have appropriate strategies and training methods. But of course, if you haven't been in the field, how would you be able to answer these questions? Perhaps I can offer some advice about what you would be looking for as a self-defense method.
1) Does it have an explicit overall strategy? Can the teacher explain it?
2) Does it emphasize footwork, flanking and moving, getting to the side or behind the opponent, like the chinese internal styles (bagua xing yi and tai chi) or boxing or aikido?
In fighting, footwork is king. You never want to be immobile or static while within striking range. You should always be moving, at every distance, and you always want to be keeping your centre on them and be moving through their center while their center points elsewhere.
3) Does it attack above and below the waist simultaneously? Are you always taking away the opponents base? An example would be to trip the opponent while at the same time choking him.
4) Is it dual handed? Are both hands coordinated to work in unison?
5) Very importantly, does the approach account for the two important distances in fighting (we assume one on one fighting for the moment).
Distance #1: close-contact (CC). This is the distance where two opponents can touch/kick each other while both are keeping their weight on the back leg. Almost all fighting systems are okayish-to-good at this distance: wrestling, boxing, wing chun, etc.
Distance #2: front foot distance. This is the distance where one of the two opponents must have his weight on his front leg in order to attack the other. This is the distance that many martial arts spar at. It is an absolute disaster to be at this distance for more than the length of time it takes to cross it. Fight/close or fuck off.
Sparring, as usually practised, teaches guys to hang around in this distance, waiting to be knocked out. Remember, the fist or even the back turning kick is often much quicker than the eye.
6) Styles that feature lots of kicking. When we kick, we become immobile, resting on one leg, and vulnerable. If you can kick, you can just as well step into a good position and use your hands, with balance, from a safer position. Most of your "kicking" should in fact be your walking. I can't really explain this here, but your movement though the opponent's center allows you to kick shit out of his base as you go through. (Knees are fine)
7) A lot of fighting is about balance, so a good style will always emphasize trying to take away the other guy's. Pull arms, rip open guards, twist their body, get their weight on one foot, etc. Note how many guys fall over, miss, have no power when they punch. Again, feet in the wrong place.
8) Once you are in there at CC, maintain contact, keep flanking, moving, but also taking the center and the opponent's base and moving forward. Don't break unless you think the other guy is going to get a grip on you and pull you down. Then quickly rejoin attack from a flanked position.
9) Does the style strike while moving or does it most typically strike from a fixed position (e.g., as in ring boxing - no flame of boxing which i am a fan of)?
10) Speed matters, but balance matters more.
11) Trains your flinch response?
12) Good guard positions? In my opinion, almost every guard practiced in popular martial arts is shit, including middle-of-the road boxing. Kick boxing and muay thai often have good strong guards. The question is, can they easily punch through your guard, around it, or tear it away, or just lean on it or hold it? This is a big issue. Hard to give a good example but maybe check out Floyd Mayweather Jr and Dmitry Kovalev and note how they "close the circle" of their arms and use the elbow. I'll stop. Too hard to discuss.
13) Is the teacher a bully or a good guy?
Ok. Enough.
Very hard thing to choose a martial art/self-defense style but the first thing I would ask is "what does this style envisage as constituting a fight/street fight?" One on one? Multiple opponents? Spontaneous explosion or "let's go outside and settle this"?
And what techniques are "allowed"? This second is important because how you train will be pretty much how you fight. If you have trained only with closed fists, then you will never, for example, slap a guy on the top or back of his head with an open hand yet this hand and this target are very often available in fighting.
Once you have that statement, consider whether they have appropriate strategies and training methods. But of course, if you haven't been in the field, how would you be able to answer these questions? Perhaps I can offer some advice about what you would be looking for as a self-defense method.
1) Does it have an explicit overall strategy? Can the teacher explain it?
2) Does it emphasize footwork, flanking and moving, getting to the side or behind the opponent, like the chinese internal styles (bagua xing yi and tai chi) or boxing or aikido?
In fighting, footwork is king. You never want to be immobile or static while within striking range. You should always be moving, at every distance, and you always want to be keeping your centre on them and be moving through their center while their center points elsewhere.
3) Does it attack above and below the waist simultaneously? Are you always taking away the opponents base? An example would be to trip the opponent while at the same time choking him.
4) Is it dual handed? Are both hands coordinated to work in unison?
5) Very importantly, does the approach account for the two important distances in fighting (we assume one on one fighting for the moment).
Distance #1: close-contact (CC). This is the distance where two opponents can touch/kick each other while both are keeping their weight on the back leg. Almost all fighting systems are okayish-to-good at this distance: wrestling, boxing, wing chun, etc.
Distance #2: front foot distance. This is the distance where one of the two opponents must have his weight on his front leg in order to attack the other. This is the distance that many martial arts spar at. It is an absolute disaster to be at this distance for more than the length of time it takes to cross it. Fight/close or fuck off.
Sparring, as usually practised, teaches guys to hang around in this distance, waiting to be knocked out. Remember, the fist or even the back turning kick is often much quicker than the eye.
6) Styles that feature lots of kicking. When we kick, we become immobile, resting on one leg, and vulnerable. If you can kick, you can just as well step into a good position and use your hands, with balance, from a safer position. Most of your "kicking" should in fact be your walking. I can't really explain this here, but your movement though the opponent's center allows you to kick shit out of his base as you go through. (Knees are fine)
7) A lot of fighting is about balance, so a good style will always emphasize trying to take away the other guy's. Pull arms, rip open guards, twist their body, get their weight on one foot, etc. Note how many guys fall over, miss, have no power when they punch. Again, feet in the wrong place.
8) Once you are in there at CC, maintain contact, keep flanking, moving, but also taking the center and the opponent's base and moving forward. Don't break unless you think the other guy is going to get a grip on you and pull you down. Then quickly rejoin attack from a flanked position.
9) Does the style strike while moving or does it most typically strike from a fixed position (e.g., as in ring boxing - no flame of boxing which i am a fan of)?
10) Speed matters, but balance matters more.
11) Trains your flinch response?
12) Good guard positions? In my opinion, almost every guard practiced in popular martial arts is shit, including middle-of-the road boxing. Kick boxing and muay thai often have good strong guards. The question is, can they easily punch through your guard, around it, or tear it away, or just lean on it or hold it? This is a big issue. Hard to give a good example but maybe check out Floyd Mayweather Jr and Dmitry Kovalev and note how they "close the circle" of their arms and use the elbow. I'll stop. Too hard to discuss.
13) Is the teacher a bully or a good guy?
Ok. Enough.