I don’t know if we’ve talked about the lightbulb trick, but I always use that on the first fire up. Basically put a lightbulb in the middle of an extension cord. With tubes removed and amp plugged into the extension cord, when it powers up, the lightbulb should stay off. If there is a short in the amp, the lightbulb with glow.
Perhaps not the most elegant trick, but it does bring a tad bit of peace of mind before the first full tube fire up.