There's no way that I know of to measure the maximum safe discharge rate of a battery, other than through experimentation. This information is usually provided by the battery manufacturer. The maximum safe charge and discharge rates are given as a "C" rating. For example, ThunderPower 2100mAh LiPol batteries have a maximum continuous discharge rate of 10C. That means the battery can be safely discharged continuously at a rate of 10*2100mA = 21 Amps. Most NiMh cells can typically withstand 10C continuous discharge, but they will get hot.

If you want to measure the current draw while the motor is running, then you'll need an ammeter that's capable of handling the Amps. There are a couple of great meters designed specifically for R/C. One of them is made by Astro Flight... http://aeromicro.com/Catalog/astrofl...er_1519029.htm