Firstly you need to decide on an area where you are consistently having failures or faults. That is one way to reduce failures without getting too technical. Next you need to decide what safety precautions you need in place, so that you can make sure you or the person performing the work understands what the hazards and risks there are. Then you go about setting yourself up for the inspection by making sure you understand what you will be looking at when you open the panel or whatever it may be that you are going to investigate.
Only then do you decide on criteria for failing or accepting certain temperatures in the equipment or environment you will be working on. These criteria, if you have no idea of what they should be, can be based on what the manufacturer specified as viable or safe temperature limits. These are available from any supplier, depending on how well you treat them.
But please, remember, SAFETY always comes first.Cowboys die.Especially where electrical circuits are involved.
As stated by the shyflyguy, safety is the 1st consideration. 1. Having the target temperature at or near the ambient temperature. 2. What is an abnormal temperature reading for your first survey??? If there are a number of the same apparatus you can gain some experience by comparing like with like. If there is only one target, over a period of time you can built a data base as long as the conditions and operating parameters remain constant during each survey.