Try an experiment, get a metal cup and put a small amount of water in it. Set the build plate to 105C or 110C and then place the cup in the center then on the corner corner and one edge. Look at the water and if it acts the same in all locations. If it boils in the center and cools off when moved to the edge, you have a problem. This is very crude but, I think good enough
I thought of this one evening when I was drinking coffee and working on some software, my V0.2 looked like a nice coffee warmer, Set the plate to 80C, and closed the door. Then I wondered if setting the cup in the middle of the plate or the edge would be best. Then thought I could test it
I bought a cheap IR "laser" non-contact thermometer. I doubt it is accurate on an absolute scale but it should detect relative temperatures. It can't see any variation across the plate.