As a testing consultant for Microsoft customers, I often get asked about several aspects of my job, like: "Where do you start looking for issues in a system?", "How do you deliver difficult messages to customers about their applications?", "What causes a test to behave like this?"
Luckily for me, the answers are easy: "I let the system TELL ME where it is hurting", "I am paid to tell the truth, so I simply deliver the FACTS", "I am not sure why it does this, but I can find out by testing it". At the end of the day, the best way to be successful is to apply solid logic and good processes, be sure everything is well defined and when you don't have and can't find the answer, then go figure out the answer. TEST your testing strategy.