If you just need to estimate how hot these batteries are getting, I do have one trick I use in our lab when heating liquids.
If something is too hot to comfortably hold in your hands for more than a few seconds, it is hotter than 60'C (140'F for those who don't do metric).
If I were going to measure temperature of batteries frequently, I'd get one of those infrared thermometers that you point at a surface and it reads the temp.