I have this tach/voltmeter from Hangar 9. http://www.hangar-9.com/Products/Def...?ProdID=HAN111
It appears to work well and places a load on the battery pack which is dependent on the voltage and number of cells in the pack. Without putting a simulated load on the battery a voltmeter by itself won't tell you a whole lot.
I thought it was a pretty handy device to have laying around.
They sell for around $45 and can be hard to find at times but I found one in my local hobby shop sitting around.