Took the idea from here and made some modifications.
Clone this repo into Assets
Add ThermalCameraScript to your camera and hook up the resource references (mouse hover each property to see details)
For any heat emissive object, add TemperatureController to it and set its temperature (manually or with script) (Again mouse over the property for details).
When the game is running, press space to toggle between normal view and thermal vision. Or edit Update() in ThermalCameraScript.cs to set up your own toggle.