If you don't have a program like Motocalc or such, hook an amp meter, or an Astro or Watt meter, in line between the motor and the battery. With the prop on, run it up very briefly and note the amp drawl. You'll know then how many amps it pulls and what you will need in the way of an Esc. Note this will only work on a brushed motor.

I did some quick checking on Motocalc, depending on the gear ratio, you may need and even higher rate Esc.