I think that it is hard to define, in some respects. Certainly there are "tells" that you can use. But that doesn't come up often, and you don't know if someone is repping a tell or if it is honest.
The dynamic of the game is much different. You get a feel for the player much faster because there is so much more information there. While not a tell exactly you can see how old someone is. you can see when someone is thinking about a call and then folding or thinking about a fold and calling. Especially pre-flop when there is less acting taking place.
I think a poker 'tell,' in the traditional sense, is a rare thing. You don't often see people licking their oreo, crying blood, or unconsciously running their eyes or whatever. But there is information out there.
I can get a feel for a live table in 15 minutes that I cannot get from an online table.
I find that people play more loosely live than they do online across all stakes I have played at. People are also more likely to see flops or call that preflop bet, especially if there has been a limp.
I also see more good players limping live than they do online. Part of this is that there is less fold
equity, and therefore you have more incentive to make that limp knowing you can call a single bet.
All of the differences are hard to quantify, but a lot of it just comes from the format. Hands are slower, people want to play, so they play more loosely to see a flop. Online hands are faster, so people don't feel as badly when they have to fold knowing that another hand is coming in a few seconds.
