Online poker rooms are probably the safest place to play at right now. Did your mother ever tell you to "never trust a stranger"? Well, playing online limits the number of sneaky people you could potentially be dealing with. The dealer is a program. Programs don't cheat. You don't have to worry about any sort of illegal card handling. There are no "cards" online.
Some people worry about hackers. It's been many, many years since the last major instance of hacking affected anyone wagering money in an online Hold 'Em game. Since then we've had breakthroughs like the SSLv3/TLSv1 encryption algorithm and multi-layered random number sequencers. Not only do we have these very technical advances, but all the ones you would expect in a real casino.
The most common form of online cheating is player collusion. That's when two or more players reveal to each other what cards they hold. This form is more difficult to pull off in a live game, but playing online gives players the chance to have a fellow player on the phone with them, or sitting next to them at another computer.
Fortunately most of these players are the ones who have no idea how to use that advantage. These two must coordinate well together, which takes practice. Online poker rooms not only look for telltale signs of collusion, but check to see if two players always play at the same table together. They would have to use "hit and run" tactics when using this form of cheating. Colluding players would have to hit up a high-limit table, then flee to another online poker room with their winnings and never look back.
To make a long story short, it is barely worth it to try it. But just in case, here are some things to look for...
1. A team of players who try to "steamroll" other players out of the game. This means reraising each other to make non-colluding players call multiple bets at once.
2. Players who never play fast pre-flop. That's the time to talk about what they hold. Watch whoever is under the gun and be mindful of reaction time.
3. A group of players who hike up bets by reraising with someone in between them. This gets the maximum amount of bets out of a third party. The alarm should really go off if one of those reraising players has a piss poor hand, or if one of them folds with one bet to go despite a massive pot. There's a time to bluff and a time to fold and most people have an idea when that is, so be on the lookout for players who exercise horrible judgement like that. Identify that person as a potential goldmine for yourself or a potential cheater.
Once again, it's VERY rare for someone to even attempt these sort of things. Still, never accuse anyone of cheating. Either contact the poker room's administrators or just get up and leave. Poker rooms like PartyPoker have around 45 or 55 tables going on at once, so you have options.
