If you have a multimeter, you could hook it up in series with the motor and see how much your motor is pulling. 20 amps should be plenty for most planes.
Not gonna work. The most a multimeter will usually read is 10 amps. Also the meter must go in between the battery and the ESC to read properly. You need to use something like the Astroflight 101 Wattmeter which will read higher currents.
The numbers associated with the motors don't mean anything useful unfortunately. Eflite has several motors whose number equates to an equivalent glow engine. whether it's accurate I don't know. Usually what you do is look at the wattage of the motor and use it to determine what you need. Trainer types and high wing floaters like Cubs can probably get away with 50-75 watts/pound, higher performance aerobatics models maybe 100 watts/pound and for 3D 100 -150 watts/pound. With brushless motors this is easier to determine beacuse the data sheet shows either rated power in watts or max allowable current. Brush type motor usually don't have this information.
Mike