Heads up one-on-one poker
The only way to insure there is no collusion happening at a poker table you are playing at is to only play heads up because single opponents have nobody to collude with.
Heads up is 1 on 1 poker where you play against a single opponent like in the image below. It is a true match of wits and you play further in a lot more hands because medium strength starting hands are more playable against 1 other starting hand than 8 others.
Prevention by the poker room
Sites might do their best to try to spot collusion in ring games (games with more than 2 players, typically 6-9) and tournaments but VPNs can fake IP address and players can verify with fake IDs.
The sites can only respond to colluders newest tricks after the sites have found out about them which means colluders are always a step ahead. This only leaves hand analyses which is a labour intensive and imprecise method that may or may not catch the collusion.
Even if a site found you lost money due to collusion there is no guarantee that they would reimburse you the correct amount if at all. Even if you did get fully reimbursed you would still have been negatively affected; you were without those funds for however long, you lost confidence in your game and your hand history and stats are all messed up.
Decentralized poker with no admin
Bitcoin will allow decentralised poker with no central site administer and no rake. People are already working on such implementations and I am amazed they are spending time working on ring games. It will impossible to stop collusion in these games meaning most people will not want to play them.
I am greatly looking forward to these decentralised poker platforms but I will only play heads up.
I also think bitcoin poker industry leaders like Nitrogen Sports (review) should be doing more to stop collusion by encourage honest heads up play, an option which is often unavailable to bitcoin poker players.