Rei already nailed it. As Rei pointed out, it is typically straightforward to simply calculate how many combos villain can have, taking into account your hand.
Another example, you have pocket nines. Flop comes 872. You want to determine how many T9 combos villain can have. Clearly there are 4 tens out and 2 remaining nines. So villain can have 4*2=8 combos of T9.
Somewhat related tangent: Where it can get a little tricky is when suited vs unsuited comes into play. Say you have pocket aces in a hand which was three-bet preflop. Flop comes out 722r. You want to determine how many hands villain can have that are beating you.
Given the preflop action you reason that the only hands that are beating you are pocket sevens, pocket deuces, and Ace-Deuce suited. He can have 3C2 pocket sevens and 2C2 pocket deuces. But you have to compare the suits of your aces to the suits of the flop's deuces to see how many suited A2 are possible.
Same principle could apply AK vs. AJ on an AJx flop in a three-bet pot. You may believe that the only AJ villain could have is AJs. So you compare the suit of your Ace to the suits of the AJ on board to see how many AJs remain possible for villain to hold.
Awesome thanks for the explantion! It is the more involved unsuited vs suited cases with multiple cards I was trying to solidfy the combintorics formula for myself to use.
