One way of telling characters apart is to have them have a favourite phrase or word. The great thing with this technique is it is highly unlikely two characters would have exactly the same phrase or word. Even if they did, they would emphasize certain words differently unless deliberately wanting to ape someone else.
Do your characters swear? Are there certain words they'd never use (so when they do come out with them, your reader knows they are really under pressure)? Do they use slang? How big a vocabulary do they have? And things like stuttering need very careful handling. Not only do you not want to upset any readers who might study, a sympathetic portrayal could do some good here.