I have an inexpensive digital thermometer in my tent that has a probe, provided to give outdoor temperature readings as well as indoor. I keep the temp probe in between and below the two light fixtures that are in there, right around the tops of the plants. This is where the temperature really matters, and where it should be warmest (of course it can get hotter approaching the bulb itself). Your max canopy temperature ideally shouldn't exceed the high 80's or 90*.
Fans increase air movement\circulation, blowing heat away and consequently cooling things down, whether it is a plant or a thermometer. Regular household fans by themselves are only going to help so much, though, and they really do just blow the heat around. They may help to blow the heat away from the lamp and equalize the temp in the room as a whole, or blow cool air into the room, but what you really want to do is expel the heat and bring in fresh/cooler air. This will cool the grow room and provide the plants with fresh CO2.
When you are using high wattage HID lighting, or using HIDs in small spaces, it helps to use vented/air-cooled lighting fixtures.