Since the Evel Knievel Stunt Cycle was a true action toy then you can bet they sure took a beating.
While the cycles had a certain degree of toughness they would ultimately become broken down at some point.
Here are some things to look for when buying stunt cycles as it can seriously degrade the value.
Broken Handle Bars: Major defect. If there is no handle bar then Evel can't ride it. Can he?
Foot pegs/stur ups: The foot pegs were a common part that would be broken. These cannot be replaced as they are molded to the body of the motorcycle. The toy can still be used since the handlebars create enough grip for the action figure to stay fastened to the bike.
Front End forks:Major defect. Make sure the front end forks have not become so loose that they make contact with the body of the stunt cycle. If you push on the forks they will touch the body but bounce back into place. If you see tire burns threw the plastic then it certinely is a poor functioning cycle.
Trick Bar: The trick bar on the cycle seat is replaceable (if you can find it). They would often become lost since they detached easily and would break easily as well. While having no trick bar does not effect the actually function of the bike it does degrade the value.
Kickstand: The kickstand is the most common broken part on vintage stunt cycles. Sometimes only one side of the kick stand would break. Either way it degrades the value but not actual playability. You will not be able to stand the bike up with out it.