as sixto has described there are times when the fan might be partially engaged without it being exposed to the requisite temperature.

The easy way to figure if the fan is freewheeling is to check it after it's been running for a bit. Pop the hood and use a rag folded up and attempt to gradually interrupt the spinning fan's blades. A working clutch will allow the interference from the rag to stop the blades from spinning in a couple seconds.

Even a perfectly working clutch will have the fan spinning most of the time the engine is running, because even when it's cold there is some coupling between the blades and the clutch, but the fan is just barely coupled and it's easy to stop with a bunched up rag or something similar, a piece of cardboard of a folded up news paper or something like that.