using just a resistor doesn't give you a stable load over time; as the battery voltage drops, so too does your current which can skew ratings somewhat.
A better way to do it would be to use a constant current load, hook that up to your battery and then measure both current, as well as voltage, from fully charged (4.2v) down to fully drained (3v).
However much current you set it to, multiply by the total time it took for the battery to discharge (if, 1 amp, and it took 2 hours to discharge, then have a 2000 mAh capacity battery) and whammo, battery capacity tester.
Other thing is, most datasheets specify capacity over different loads. Every li-ion battery will have a smaller useable capacity the more current you draw from it. It's generally useful to have a datasheet for the specific battery type you're testing, but if not, I'd say somewhere between 500 mA and 1 amp is a good place to start for most.