Well you can almost get away with no test equipment. You need a battery charger that lets you know how many mAh you put back into the pack to charge it. Take a fully charged pack to the field. Fly for 5 minutes the way you normally do and measure the current put back into the battery. Divide the current by 5 to get the current drawn per minute and size your pack accordingly. I would add a fudge factor of 20% for safety.
John