Of course, you can never know exactly what your opponent has until showdown, but you can gather information throughout the hand. For example, if UTG open raises he is probably playing around 10% of
hands, which is 66+, A9s+, KTs+, QTs+, and suited connectors down to 98s.
So, if you have 99, you are already ahead of much of his range.
If the board then comes A-Q-T, you need to proceed very cautiously, because the board hits his range full in the face with straights, sets, 2 pairs, even bottom pair now has you beat. On the flip side, if the board comes T-5-3, you can be reasonably confident you're ahead. Sure he has some 10's in his range, but the
odds are he doesn't. I would tend to call and check, or bet small when he checks to try to get him to call with hands like AK, AQ.
The important thing is to not get carried away with medium pocket pairs. A lot of players get excited when they have a pair in their hand and don't seem to recognise that if it doesn't improve, they just have a very marginal made hand. Oppositely, when players do hit their set, they think it's time to get clever and slow play their monster, when actually you want to be building the pot to get the most value out of your premium made hand.
Good luck in your games.