There is no particular way to identify someone hitting a set. usually continuation bets cause your opponents to fold. but in case the opponent does not fold, check the texture of the board and see if there are any draws which your opponent might be waiting else there is a possibility of a set. If the turn or river does not improve your hand further,keep the pot small, checking behind is not a bad thing.
Check/raise either on the flop or later street usually a sign of monster hand such as three of a kind but some good players also would take the opportunity to bluff..
How to recognize poket pair of opponent in late stage. Or how to recognize opponent hit three of a kind.
No real way to determine. As the tourney or game progresses you just need to be able to read each player and what they raise with pre, slow play you on, etc. I am more conservative at the beginning of a game because I want to be able to read the players quickly.