I'm in agreeement with Ayrton, stay out of the way of any possible blast (stand to the side), treat EVERYTHING as if it were hot, and get a good quality tester.
GE taught me to test "HOT-COLD-HOT"; it's good advice. Check your tester on something that you know is energized (to make sure it works), then test the conductors to make sure that they're dead, then go back to the energized conductors to make sure that the tester didn't pick that particular instant to break. It only takes a second and it can save your life.
