Not sure if a NAPA would have a temp gun to use or not. I'm sure they sell them, but they may not have one to use. Autozone is usually better at doing stuff like that. Shoot it at the thermostat housing (if it isn't chrome, reflection off of anything shiny can give a false reading). The temp will be a few degrees hotter than the reading your get off the thermostat housing because your only reading surface temperature, but it'll be close. If you have a chrome housing shoot the water pump and/or timing cover. That's a good close second.