After you’ve played at a poker room for a while, you’ll probably start running into the same players, particularly if you consistently play at the same time of day. Here are a few characteristics you can watch out for:

• Same players always at the same table
• Same players always in the hand together
• Extremely aggressive two-way betting followed by a fold at the end
• Always/never chatting
• Chatting seems overly formulaic

If in doubt, report the hand to the room’s administrators along with the names of the players you think might have been colluding. The rooms take this sort of thing very seriously and will examine everyone’s down cards and betting patterns to see if anything is amiss.