The thing about online poker is that there is no speculation involved. Objective proof is easy to come by, and if any site failed this easy test it would be destroyed.
Plug 50,000 hand histories into one of the commercially available programs, or one you make yourself, and it will spit out incredibly mundane/expected/normal numbers. If something is expecting 8.237% of the time, it will have occured 8.2+/-%.
What I think most people fail to understand is that the best way to make money in the poker world is to run a fair game. There is simply no comparison.
The main point though is even more imporatnt. If a cardroom were to cheat, the way they would do it is to deal out miserably bad hands to everyone. Since the rake is capped, they want virtually no action. In a 5/10 game the rake gets capped just from one player raising preflop, betting the flop, betting the turn and betting the river, while getting called by one single opponent from the big blind. Any more action than that is bad for a cardroom. Dealing out multiple good hands would be idiotic for a cardroom.
Cardrooms HATE action and big pots, especially No Limit action. They make zilch off of it.