How can you tell if they are bad?

 

Lift up the dust cover and check to see if there is any fluid on the top of the strut. (fronts)

 

when they dont work

If there is an oil leak. I say LEAK not seepage or sweating, your looking for actual trails of oil on the strut, not dust collection around the seal.

If the piston/valve fail/become damaged youll no longer have proper damping, youll likely get a bouncy suspension. The old school rule of thumb is to bounce the car up and down, when you stop bouncing it the car should stop moving within one up/down cycle.

 

good question, and straight answers. thanks to all below!!!!