Ambient temperature is just the temperature in the vicinity of the computer. If the computer is in a corner, under a desk, or anyplace with restricted air flow, ambientwill not be room temperature.
I use an electronic cooking thermometer. It goes from -45 C to 200 C.
Another option is a Rat Shack indoor-outdoor thermometer. Use the builtin "indoor" part to measure ambient temps. You can use the outdoor inside the compter case to look for hot spots.