Identifying the key defender can be a very useful technique, particularly when you are in attack. For example in this position (taken from The Encyclopedia Of Chess Combinations):
Here the queen sacrifice on f3 look very promising (it is often good to start your calculation with forcing moves as your opponent won’t get time to execute his plans) because it opens the g- file and increases the scope of bishop (f1-a6 diagonal) and knight. Then I calculate ….Qxf3, gxf3 Rg8+, Nxg8 Rxg8+ and notice that he can return the queen by playing Qg4. Even after Kf1 Ba6+ he can return the material. So I rejected that variation and looked around for something else, and note that Black has to do something as otherwise his own position is critical.
However we can conclude that the queen is the key defender if I want to play Qxf3.
With this in mind I looked position again and tried to see what happen if queen was not on the d-file or the h3-c8 diagonal. Now it is crystal clear you can deflect the queen by playing …Rxe7!, again this is a forcing move. It is also wise to see whether your opponent has a good intermediate check before executing this kind of sacrifice.
Now you know how to try to solve such positions.