[MetroMPG]

If the fan is coming on, more air is needed. That's why I wondered if you could tell.

The fan draws a LOT of power. Probably as much as running with full lights on, I'd say. That's one reason (aside from additional stress the added heat puts on things) to size your block to avoid having the fan come on in normal driving.

If you hunt around, you may be able to find a thread about how to hook an LED up to the fan circuit so you can monitor it if you can't hear it.