Try useing your temp gun on the front of the radiator unless you have ac then you need to do this on the back of radiator
car needs to be at operating temp
Your looking for hot and cold spots in the core so your going to have to move it around top to bottom side to side
big temperature changes mean the radiator has some issues
Blockages not flowing right thru the core
You can also use temp gun to check upper and lower hose temp
Lower hose should be cooler than top