Since you're never going to be able to get a direct shot at the coolant you're always going to be reading a surface colder than the liquid beneath it. At best you could try the engine outflow hose (top one) or the first few inches of the radiator. Either way even if you get your engine up to 220 degrees you're not going to read that with the temp gun. And provided you're cooling system is in good shape you'd have to disconnect the fan or go for a seriously extended boosting session to get it that hot (near track like conditions).
Best way about it: Just get an aftermarket gauge if this is going to be something that you're going to be concerned with continually.
1989 240SX Fastback. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1995 300ZX Twin Turbo Warning: Objects in mirror aren't as fast as they thought they were.
