Couple things...Easy to figure out once you are looking at the joint. As you may or may not know the u-joint consists of 4 end caps and a 'cross' or 'spider' in the center. Inside each cap are needle bearings that roll against the 'cross'. What happens when the joint is bad is these roller bearings are pulverized.
When you look at a u-joint, you may see that the inner portion of the spider just below where the cap sits looks greasy, and on other caps, it may look rusty. Chances are, if it looks rusty, the needles are gone.
How else to check. On either axle shafts, or drive shafts, there should be NO PLAY between 2 shafts that are connected by a u-joint. If you turn shaft 'A' by hand, shaft 'B' (connected to shaft 'A' thru a u joint) should move exactly the same amount at exactly the same time. Also, in the joint itself, as you move it, it should have NO RELATIVE PLAY in it. In other words, if you see the 'cross' or 'spider' moving, you do not want to see one of the caps wobbling on the cross, everything needs to be moving the same amount and smoothly.
Now, the double cardan u-joint (2 u-joints right next to each other) found on the front drive shaft, is another story. It is possible for one of the u-joints to be bad and for you to not see any difference in relative motion from one shaft to the other. If I were replacing other u-joints, I would probably replace these as well, just to be on the safe side. Do a search on that joint though, as there is a centering ball and spring in there and it is kind of like a chinese puzzle to take apart and put together. I've done it, as others have, but I'd have a hard time describing exactly what I did in what order...HTH....
