Charge .5hr labor more at least. If you can't see the marks fail it for mechanical. There is one around here with bright white marks and the owner shows up with the cover moved already, I like that guy. I've tested some that I think have never actually been checked before, the cover is rusted shut and I can't see the marks even crawling in the engine bay.
For generally having the easiest cars to test, Toyota does have some of the trickiest crap to test.