unless you need the exact temperature, just set the emmisivity to .98 and do your survey.
what you may really be interested in is the Delta T of like or adjacent components.
if you really want the exact temp install thermocouples or use contact temp probes.
as for a stick on "label", just use standard black electricians tape, not the shiny plastic stuff they use no-a-days, it has an emmisivity of .98.