Nickel metal hydride (NiMh) batteries can self-discharge as much as 1% per day. So 30 days after they are charged they could be discharged by 30 percent. NiCd is not as bad. Lithium batteries hardly self discharge at all. Charge in November, fly the next spring.

Harbor Freight sells their multi-meters (to check volts, current, resistance, etc) for $1.99 most every day. that particular item is almost always on special price. You need to put some "load" on the battery to get a better reading.
So I solder a resistor in parallel to the meter. You could also solder in a small light bulb (like a automotive tail light bulb). If you don't want to make something yourself, then get a "loaded voltmeter" from the hobby shop.

Something like this is fine: http://www3.towerhobbies.com/cgi-bin...?&I=LXWDZ5&P=7 for $8.99