How will I know if the TVS has done its job and protected the circuit board?
The engines keep running.
After a "spike" will the TVS be rendered useless, and in need of replacing?
No, the TVS diode is good for pretty much infinite spike suppression as long as it's overall ratings aren't exceeded.
How will I know if the TVS needs to be replaced.....does it get that "burnt component" appearance?
You won't know if they fail open. However, the most common failure mode is shorted, that you will notice!