Spotting attacks is the basis for defending against attacks. The only way you can actually defend against an opponent is to see his angle and you don't win a game where you only "defend" unless your opponent blunders.

In chess there are two possibilities that give you an advantage material and positional advantage. If you plan on "defending" your position you most likely want to protect your pieces and don't leave them undefened, but that is not enough since you have to take into account forks and binds and more. If you want to play defensive you can try placing your figures in a more defensive style (see hedgehog or similar), but that is not something you can derive chess problems from. So long story short. Chess problems on defending are in essence the same as the attacking once. You have to find the actual threat. If you want to be better at defending just take the time and try to understand what your opponent is planning as well. It is the same thought process in the end and you will get better with time.