Right on with the RNG. It isn't as completely random as physically shuffling and dealing a real deck of cards, but it is the closest thing possible in affordable technology. Of course, as with any program, there are varying levels of intricacy based upon what a given poker site can afford to use. If you go to a particular site, they will usually have a section in their FAQ's which addresses their RNG. pokerstars, for example, claims to have a patent in process for their RNG program, which they say currently produces the largest number of permutations in the industry, making theirs the self-proclaimed "most random deals on the internet". Newer, smaller sites may be a bit more suspect as they don't have that kind of capital to invest.
The debate rages as to whether ANY poker hands dealt online are truly "random". The detractors say that it's obvious some "salted" hands are inserted into the mix to set up entertaining, dramatic showdowns. I'm not one of them, as the sites have nothing to gain from who wins or loses a given hand. They take their rakes from the ring game pots and add juice to the tournament buy-ins to cover their end. Don't see why they'd bother with staging hands.
When you think about it a real pack of cards thats shuffled isn't really 100% random either. I would even go as far as saying that the rng used online are probably more random than any hand dealt offline.