I have a FLIR thermal camera that comes as an iPhone attachment. FLIR ONE for iOS Personal Thermal Imager
https://store.apple.com/ca/xc/product/HL5K2VC/A
It works well, and I used it to diagnose a faulty chip on my board at our latest PARC (peel radio club) buildathon, by comparing a working board with the nonworking one.
if you want to try something different, I wonder if you could get some thermochroic paint and dab some on the transistor so it glows red above a certain temperature.
