I know nothing about this, but I am just going to throw this out there. I would think that the reason the mechanic tested the exhaust manifolds at startup is because that would be the only time you could put your hands on them. With an infrared thermometer you could read it even at full operating temperature, and still detect a difference, if there is one. Now that we have been talking about manifold temps, I can remember seeing mechanics doing something similar (hands on pipes) to motorcycles to see if they were getting a misfire or had carbs out of adjustment.