An excellent tool for testing a theory is?

An excellent tool for testing theory is verifying deductive principles

which mean that they keep verifying the principles that may prove the theory, and eliminate them if it failed to prove anything. After the eliminates all the impossible, whatever the theories remain, than that theory will be seemed as  true :) 

An excellent tool for testing a theory is the scientific method. Create hypotheses that would come from a theory. Gather data about these hypotheses. If the data is true and the hypotheses are confirmed, then it's likely that the theory is true as well.