A couple of years ago I played a lot online, and I mean a lot, but never for massive stakes, and in the end I lost all my winnings for the sake of drunken laughs… Anyway on the major sites I think you can trust the proceedings, but always be vigilant. Obviously with online poker you can’t pick up any physical tells, so remember the only way to read an opponent is by patterns in play. Pay close attention to EVERY hand, look at the previously played hands function to see what people had, and make notes on players you see (over a few weeks, you’ll start to see this pay off as you often end up in games with the same people). They don’t need to be extensive notes, just comments on how loose they are, do they use pot odds, do they prefer to slow play, are they easily bullied on the blinds etc. Paying this close attention will also help you identify the rare occasion when you have players colluding at a table, which is the main risk you face online imo. When it happens it’s usually obvious, and you should report these people even if you there’s an element of doubt.