For brushed motors you use the 5-second rule, if you can put your finger on the motor for 5 seconds, it's fine, if you pull your finger away in under 5 seconds, it's too hot.

And if it's too hot, don't bother trying to cool it, gearing down is the only way. With brushed running too hot is a symptom (of overgearing), so solve the root of the problem rather than trying to cover it up. You can add the heatsink afterwards to keep things a little cooler, but don't depend on the heatsink to make a hot motor run in a 'safe' range.