What to look for in a MC29 going bad?

What are some things to look for prior to a motor controller going bad? You know, this guy

http://www.vexrobotics.com/media/catalog/product/cache/11/image/296x/5e06319eda06f020e43594a9c230972d/2/7/276-2193-motor-controller-29-a_1.jpg

I know how to test for catastrophic failure, but how do you test to know your MC29 is on the way out?

Do motor RPM’s drop? Can we test for resistance or amperage going to the motor being different? Temperature out of whack?

Does it act inconsistently or anything like that or does the MC29 just work pretty well until one day it does not and decides to bust?

I have never had a motor controller that was broken and didnt have a physical sign of abuse.
Something like damaged wires or big hole showing it was shorted.

I am trying to detect degradation prior to the catastrophic failure event. Nicked wires is probably not detectable in the signals unless a few jumps of current cause something that looks bad on a scope in one way or another.

I guess I always viewed it was fine and then broken. Shorting a component and running way too much current through it isn’t an over time occurrence, it happens and then the component is broken. I never really noticed a controlled that I thought degraded and look fine. Usually it’s “woops I ran motor controller into a gear, woops I broke a motor and shorted the controller”.

I would assume if a controller was broken and didn’t have an obvious flaw it is because of degradation and might have been performing at some intermediate state for a period of time.

I’m not an expert on the components in the MC29, but do they change over time at all? I have not heard of any break or stop working for reasons other than damaged wires and shorting that causes the magical smoke.