What are ways to know if a verb is irregular?

I know that in spanish there are few irregular verbs compared to other verbs however how do you know if it's irregular? does it also stay in irregular form when conjugated in past,imperfect past, future,etc? i asked my teacher and other people and they say "you just got to remember" which doesn't help if i can't identify them in the first place -_- thanks for any answers! =)