First off, why do you think one may be bad?

The first thing to do is get a buddy or a girlfriend or someone to sit in the jeep and slowly move the steering wheel back and forth while you lay under the jeep with a flash light and look at every place your steering can move (ie every ball joint and tie rod end). If you see any relative motion between the part then you have a bad joint. If you jack up the front end, you might miss something. After you do that, jack up the front axle put it on jack stands, and have someone (probably not a girlfriend this time) grab on to a wheel and try to lift it up, push and pull in the top and bottom of the wheel, while you lay under the jeep and look at the wheel ball joints, again you are looking for any relative motion between the steering knuckles and the axle housing.

Sean