Wouldn't it just be the one not giving off the same amount of heat?
Chips can fail in a number of different way. There is simply no binary state for this (1 for working, 0 for not working). For instance, chip can give out same amount of hashes therefore showing similar heat signature, but producing only invalid shares? I don't have access nor funds to get high-res thermal camera!
I'm not sure heat signature is the way to go to detect faulty chips. However, if that's what you wanted to do, I think a simple laser-guided IR thermometer would suffice. They are not very expensive, you can even find them on Bitmit.