Cover the top of the thermostat with a rag and tap it lightly with any rubber coated screwdriver handle. It should come loose. At normal ambient temp, it should be closed, you should not be able to see any daylight around the thermal element and the outer ring. If you do, toss it a get a new one. If you don't, toss it and get a new one. They are inexpensive, good insurance and anyway, who wants to mess around with thermostat testing. If the part is doubtful, replace it. Peace of mind, confidence and a happy engine will surely follow..
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.