[hachkc]

Short of hard evidence (overheard conversation,video evidence,etc), I can't think of easy way to do it. As somone mentioned, you could try an keep stats of how they play to give you an indication of it but that's difficult to do and doesn't mean much with a small sample. If you are gonna accuse someone of it, you better have a damn good reason and some backup especially if you suspect multiple people of it. Best thing to do, don't invite them back or find a new game.

Also, they may have been softplaying each other which I define as slightly different then collusion. I'd define collusion as a planned and active cooperation against other players (raising against each other to force players out, chip dumping, etc) while soft playing is more of an unspoken agreement to not get involved in hands against each other (checking hands down instead of betting/raising). Neither are good but one is worse than the other.