Cheap flir for your celly telly. We use them at work to check to see if parts are over heating and where.
https://www.amazon.com/FLIR-One-Thermal-Imager-For/dp/B0728C7KNC/
Flir one only costs $200. The better one is more but if you're just trying to see what's hot this does the job fine when you hold it close.
I can see why you'd want a phone with it built in though. But there's also the Catphone models to consider, they're not too much more and I think the specs are better.