How to check a refer cooling unit for blockage

I understand that this blockage is a slow build up over time and understand how to check to see if the refer is totally blocked. What I was trying to figure out is if temperature measurement could alert us if the cooling unit is starting to have blockage. I would think the blocked areas would be cooler than the rest of the tubing. I check my radiator this way to look for blockage and it shows cold where radiator starts to develop blockage.