Since the original question was somewhat ambiguous, I should clarify:
If you are talking about recognizing different styles being demonstrated or practiced, then it's really just a matter of being familiar with the styles in question. I can identify a lot of different styles in general, but would probably not be able to differentiate between two closely related karate styles the way that practitioners of those styles might be able to.
If you are talking about recognizing different styles in the context of a fight, that's a whole different matter. You can refer back to my previous answer for that one.
If you are talking about recognizing different styles being demonstrated or practiced, then it's really just a matter of being familiar with the styles in question. I can identify a lot of different styles in general, but would probably not be able to differentiate between two closely related karate styles the way that practitioners of those styles might be able to.
If you are talking about recognizing different styles in the context of a fight, that's a whole different matter. You can refer back to my previous answer for that one.