The heater elemints are isolated from the actual block they are mounted in. So you will not feel much change in temp of the outer block.
Your grid heaters are about the same as a 1st gen. You can amp test them. A 1st gen pulls around 100 amp load when engaged.
You also can use an OHM meter. Being a resistence type hearter elemint. You will not see a high OHM reading. It will be towards the lower end of the scale. I couldn't find my spare grid heaters to test. You would most likely see less than 100 OHMs across the terminals.
2014/93 W350 DRW, fuel pin turned, 6X.018" injectors, 4" exhaust, 3" air intake horn, Super HX40/16, G56 SBC Con-OFE, 3:07's axles, 2" skyjacker lift, air ride rear suspension,B&W flatbed