fan clutch?

[jbashaw]

So i was pulling my featherlite trailer the other day with my jeep and the temp gauge went up so i pulled over and looked under the hood and the fan wasn't turning very fast.. That's the first problem.. Second is i got in and started it back up and the gauge went back down to normal... So i suspect the thermostat but I've never diagnosed a fan clutch. How do you tell if they are bad?