If there is play in the joint, it is bad. If you see red rust stains around the seal area, it is bad. If you have the driveshaft out, and the joint hangs slightly, it may be bad(rapping the flange with a hammer may reset the joint) if not, it is bad. My advice, replace them. Joints are easy to change and cheap. The cost of replacement probably less than the flatbed ride when they break.