Hey Ryan, welcome to CC!
The key to online reads is to pay attention to betting patterns.
Preflop, does this guy raise a ton of hands from whatever position he's sitting at, or is he very tight? As an example, if you watch me play FR (full ring) and pay attention to how frequently I open utg, you should be able to infer that I'm not playing much junk from there, so you shouldn't expect me to show up w weak or speculative hands if I open utg. If you pay attention to how frequently I open the btn, you should be able to infer that I'm playing a whole lot more hands than just pp's and AK.
Postflop, does this guy bet draws on the flop? What does he show down when he check/raises? What does he show down when he donk bets the flop? Is he passive or aggressive? Does he only go to showdown w the nuts? Does he bluff?
Example - I'm playing against a very tight pf player who is extremely passive postflop, but when he calls the river he's always got the nuts. So, I raise pf and he calls from the blinds. Flop is 962r and I've got an overpair. He checks flop, I bet, he calls. Turn is a blank. He checks turn, I bet, he raises 2.5x my turn bet size. Since he's passive and tight, he's not going to be raising a draw here and he's not raising Tx. Chances are he's got me beat w at least two pair, more likely a set, and my overpair is no good at this point. (fwiw, this is an example of what's called the Baluga Theorem, that you should fold your top pair hand to a turn ch/r)
Example 2 - same hand, same board, same actions, but villain is aggressive and plays a lot of speculative hands. Here we may be behind 2 pair or a set, but this villain may be raising a straight draw or an overpair worse than mine (less frequently TPTK). Now a call may be more feasible (but may still be incorrect, depends on the villain).