[suicideneil]

Yes, but it aint cheap.

You can buy something called an EagleTree Data logger- it basically plugs in between the battery and esc, and measure the current ( amps ) and voltage that the motor & esc are drawing. With the PC software that comes with it ( or bought seperately, dont have one myself so not certain ) you can plot a graph of a run, and then by cross referencing the voltage droop of the batts vs the current being pulled at any one point, you can determine quite accurately the actual C rating ( just need to determine what voltage level droop you deem acceptable under load- 3.2v per cell generally is agreed as fine ).

They cost about $100, so for some people like racers or testers its a worth while investment, but for the occasional basher... might be a bit too much money to pay for something you wont use very often ( just as well my penis came as standard... ).