When you say "put them through their paces" are you intending to build some sort of mechanical test rig, with a way to apply a force, or even a brake to them?
Have you got the datasheets for the motors and servo's so you can, at least, get some estimate of what their stall current is?
If you hold a small DC motor stalled for an appreciable time (more than a few seconds), then it may be permanently damaged by the heat (depends to some extent on the motor, sorry to be vague). With either a motion encoder, or current monitor, you should be able to detect a single motor being stalled. As you probably know, model servo's are usually DC motors.
I saw a delightful test rig a couple of weeks ago.
A guy had three identical motors meshed around a common gear.
1 - the motor under test
2 - a motor used as generator, so he could measure the amount of energy, and detect a stall
3 - the motor generating the force against motor 1.
So he could monitor whether the motor (1) was moving, and by how much, by measuring the output of the generator (2), and apply a braking force with a motor (3).
He could control the whole thing from a single microcontroller with two sets of H-Bridges.
You could build a similar mechanism.
Put a small (low thermal inertia) temperature sensor on the motors too, as a backup measure. Thinking about it, this might be useful anyway to characterise the motors (depending on what the end goal is).
You could also make a pretty good motion encoder for about a $1 with a Hall effect sensor and a magnet. That would give a good way to measure speed, and detect a stall.
HTH
GB