Sometimes you can infer the count of a suit, particularly a major suit, based not only on what the opponents have bid but on what they haven’t bid!
For example, say the bidding goes:
Say a spade is led and dummy has the 
Q 5 and you the J 10 7. The opponents have 8 hearts and neither one have mentioned the suit. Strange. Surely if either opponent have five hearts the suit would have been bid, you can asume hearts are 4-4, you can infer that East has at least five spades. If East have four hearts and four spades, East would have responded one heart not one spade. Count the East hand, you already know 9 of this player cards!!!
Now consider this bidding sequence: 
Say a spade is led and East turns up with four spades. You can infer that East cannot have four hearts or else East’s original response would have been 1.
Try this one: 
Dummy has three hearts and you have three hearts. How are the remaining seven hearts divided?
Surely East must have four and West three. What else could it be?