I use a infra-red lazer temp gun on my 60 & 90 heli's (Stanley brand sold at Sears). I usually spend the first tank full of fuel tuning in the engine and trimming and as soon as the heli is spooled down from the first tank, I measure by pulling the canopy off. It only takes a few seconds and I put the lazer dot right on the glow plug. Depending on humidity (which lately has been as high as the temperature), I verify if my tuning is close to the preferred 200 to 225 degrees farenheit. At that point I make a final adjustment.

Note; Just because the temp of the engine head is where you're adjusting to, it might not always be the optimum flying setting. I've tuned to get the best flying performance and found the engine was in the 240 degree range because it's hot and humid. Left the settings alone, came back that evening with the same exact settings and got temp reading of 210. So there's many factors, but it's better than tuning to a meltdown and by checking the temp this won't happen as easy.

Mike
Raptor 60 & 90
OS Engines
Futaba Electronics
Cooper 30% Fuels