Welcome to Arduino!
I have just done a similar project, making a 2 wheeled robot using the Adafruit motor shield. See the details at http://www.robotliving.com/building-robots/two-wheeled-robot-2.
What you need is the spec sheet for the motors and look for somehting called stall current.
My motors have the following specs.
Gear Ratio 1:120
No-load speed(3V):100RPM
No-load speed(6V):200RPM
No-load current(3V):60mA
No-load current(6V):71mA
Stall current(3V):260mA
Stall current(6V):470mA
Torgue (3V): 1.2Kgcm
Torque (6V): 1.92Kgcm
Size: 55mm x 48.3mm x 23mm
Weight:45g