The only way to test my inexpensive, probably
Amazon purchase, that I came up with is your forehead temp. I just checked mine against the medical IR thermometer. The Bayer thermometer read my forehead at 98 deg, right about where it should be (if I'm not sick!), the "industrial" IR read 94 degrees.
So thats 4 degrees of proof right there. Like Steve said, the surface you are getting the reading from can make a difference. Mine is a little finicky on a stainless pan, but a layer of oil settles the readings down. Pretty solid on a cast iron griddle though.
I pretty much use it to see if a pan is in a good temp range to start cooking something, for the most part. I'm not sure what else to use it for.
I played around with it on my wood stove to prove those little analog wood stove thermometers are wildly inaccurate. But in the end, it doesn't matter, I've learned that whatever actual temp it is, the magnetic analog one on the stove gives me all I need. If its 40F or above outside temp, I know I need to aim for keeping the stove at what that thermometer reads as about 300 or the stove will run you out of the space...need to open windows and doors to cool it off. If it's below 40, 350...below freezing, 400, in the 20's...450, teens...500.