I have a small infared temp gauged that is meant to check temps on nitro rc cars and rc planes. They are about 25 bucks. Hold it 2 inches from the top of your plants...presto canopy temp. great tool. Made by Duratrax . It's called Flash point. Get one at your local hobby shop.
yea homedepot sells infrared thermometers as well. you can also pick em up at a lot of kitchen supply stores. problem is that it doesn't necessarily measure ambient temps at your canopy level, but rather the temperature of the leaves. but it's pretty close anyways. I always just hung a thermometer from my light, it also acted as a gauge with which to maintain the correct distance from the top of the plant.