I know it must have been posted before but i'm new here and i want to tell you from my own experience some tips about how to find if someone is a thief. That's just some that i have experienced i may not represent ALL of the above but it may help.

1- First enter the game and say; any1 have weapons to give to me ?
2- Join you in the adventure and run like an idiot toward the crates and take everything.
3- Never help you when you're in danger.
4- Don't talk much. Maybe they don't want to steal new friends.
5- Never help for the boss, healing themselves with trimates.
6- Take your weapons and leave the game (if he does that you're sure he's a thief

The list may not be exact but if you want to correct something(remove/add) just post.

PSO Name: Khabal lvl 78 for now