Source: Tips For Card Play : Card Reading
You are in 4
. West leads the 
7. How should you plan the play?
Since you have three losers in the minor suits, you cannot afford to lose a trump trick as well. The normal play in spades, if you have no clue as to the location of the queen, is to play the ace and king and hope the queen falls. That is the best mathematical play but bridge logic will often guide you to more frequent success. If you know which player holds the queen, the problem is solved.
Sometimes it is possible to work out where the missing cards are by counting points. Count the HCP in dummy and add the HCP in your hand. Deduct this from 40 and you have the HCP held by the opponents. You may be able to work out which opponent has all or most of the missing HCP from the bidding or absence of bidding.
Here are some guidelines :
1. Assume that a player who opens the bidding holds 12 HCP or more.
2. Assume a player who fails to open the bidding holds below 12 HCP.
3. Assume a player who was too weak to respond has below 6 HCP.
4. If one opponent has pre-empted, the critical high cards missing in the other suits will usually be with the partner of the pre-empter.
In the example above, you have 24 HCP and East-West have 16. If West opened the bidding and East turns up with the 
A, the missing points including the Q should be with West. If West opened 3, play East for the Q, while if East opened 3, play West to have the Q.