Bad as in gone, or bad as in not too good? When they're shot the car will float around on the suspension and not feel very stable. You can look for fluid leaks, listen for noise and push down on a corner of the car, but that may no mean much. One great test is to let someone, who has a similar car to yours, drive yours. If they say, "This handles like a POS!" you need struts.
Any time after 60k miles you may need struts.
