Go w/ Batfish on this one and purchase an IR temp gauge (should be around $25). As they aren't 100% accurate you should take a couple readings and use the average.

If you're running the gearing that you want but still need it to run cooler I suggest a DC fan (computer case fan, I use an 80mm fan (others use smaller fans)), thermal grease (I use Artic Silver 5), and a heatsink.

Prices are approximated
$25 temp gauge
$25 heatsink
$15 fan
$9 thermal grease

I was hitting somewhere north of 170f before adding the aforementioned items, now my truck runs 125-140f (140f when batteries are drained)...even when ambient temps are 90f+.

Keeping the temps down will improve run time and motor life.