You can get a thermal-reactive film thermometer at most drug and discount stores. They are about an inch wide and three long. They are designed for little kids- put them on the forehead. The only problem is, if you are outside, any chilling of the skin will play with your readings. But I would say, hypothetically, that a reading taken in, say, the armpit would be just as effective.

I know we used to have one around here. I'll see if I can find it, and see if what kind of results I get from using a body location other than the forehead.