A really cheap motor

Something available at Dollar Tree, a dollar store:



For $1.00 (US) you get two electric toy cars. They are a little bigger than a Matchbox car. Here is the motor:



The little controller holds two AA batteries. The two buttons are for forward and reverse motion.

A little boy that I know has had several of these cars. Usually the wires get broken or the little buttons wear out from play, but the motors seem to have a longer life than the rest of the car. Sometimes the cars are missing the black rubber tires from the factory, but quality control for the motors seems to be consistent.

Is there a way to test the strength of this motor?

Don't use them, the motors/gearmotors don't have much torque to be of any use in a RepRap, except on the Tamiya Extruder. The brushes will also break sooner, plus all the EMI that will come out of it.
If you go with the Tamiya extruder, you'll get the dc motor with the universal gearbox anyway.

Just my personal opinion

How's the performance of the Tamiya extruder compared to others?