An iron split manifold may retain enough temp to be clocked with an IR gun, the alloy intake cools too fast to get actual data - any remaining fuel will boil off and use up the BTU.
But... it may show a trend. If the ends (1-2 and 5-6) are consistently hot, and about the same, you know you have something.
Remember that a mixture diff may be at idle only, high vacuum only, or WOT only, or any combination, so try to load it specifically where you want before taking a read. The usual plug check applies: after enough time at your test conditions (several minutes), pull the clutch in and kill it in neutral, then coast to prevent false reading.
