And how the hell are you supposed to now if someone is THAT bad (and yes, I'll call it bad since it really doesn't take much to simply read the info on the item)? Most people can manage a simple task like that, so it IS more than fair to assume that anyone rolling on what they shouldn't is a ninja (and they are - they're just not aware of it). Like I've said multiple times though - IF it turns out they made a mistake and fixes it I'll forget about it. This isn't a court of law, so "innocent until proven otherwise" doesn't apply.
I don't know of too many ninjas who stick around to finish a FP after doing so. Most just log or leave group.
As to knowing what relics do...I didn't figure it out until early 30s