I have a JR TX and would usually have the stock 600 or 700 mAh pack near death by the time I had 4 or 5 flights on a fresh charge, I have since switched to a 2200mAh Lipo pack and have had no trouble at all... I have not gotten it anywhere near dead and could probably get 3 or 4 days worth of flights at the field before having to recharge. The JR transmitters show the realtime voltage of the pack making life easier as well. As for the RX packs a loaded volt meter is the way to go. I have a digital meter that accepts bannana plugs so I simply made a loaded lead to plug into my RX packs. I believe it is a 2 watt 10 ohm resistor that I used which gives a 500 mAh load on a 5V pack. A little heat shrink tubing over the resistor makes for a nice clean lead.

Edit: Use a 2.5 watt or greater 10 ohm resistor [8D]