The choice of measurement method depends on your thermal design. If you have the top of the FET exposed, then yes, a thermal camera is a good idea. Use non-reflective black paint on top of the FET. Yes, you are correct, need to be careful of reflections. A paint to use: Testors flat black enamel, 1149. You can also use a paint pen, Testor 2549. The pen is convenient, but the 1149 paint has been well characterized. You can also use Scotch Brand 88 black electrical tape, which has a reported emissivity of 0.95 to 0.96. An EPC apps engineer said of Testor 1149 paint: “I usually apply it with a toothpick, and it takes a bit to dry.” If you wish to use a thermal probe, then you can glue it to the side or top of the FET. The junction (the section that generates the heat) is the bottom of the FET, the side that is very close to the board. So, perhaps a probe on the lower part of the side would give a good result. It sounds like you don’t have a heat sink on top, so there may not be a big thermal change between the junction and the top of the FET.