The CV -- or constant velocity -- joint is a relatively simple piece of hardware tasked with a complex job. The original CV joint was a double-Cardan -- an arrangement suitable for driveshaft and heavy-duty applications, but not light or flexible enough for use on everyday drivers. Inner and outer CV joints display a number of different symptoms; the only really difficult part is determining whether the CV failure is on the inner- or outer-end of the axle shaft.
CV Joints and Failure
Most think of the CV joints and axle as a unit, since that's how they come as replacement parts. On one end of the axle shaft is a complex Rzeppa CV joint; this side faces toward the wheel, and allows for a broad range of movement while steering. The inner CV joint -- the one next to the differential or transaxle -- is a simpler but sturdier tripod design that allows for a few degrees of movement. As a CV joint ages, the metal bearings inside the Rzeppa or tripod joints wear away. This opens up some clearance between the bearing portions, allowing them to hammer together and eventually break.
A large, smooth parking lot or freshly paved section of road are the best places to check for CV joint failure. Minor imperfections in the road surface can cause your suspension to bounce, causing a bad shock absorber, bad spring or loose suspension component to exhibit many of the same symptoms as a bad CV joint. Under many circumstances, a bad wheel bearing will mimic the effects of a worn CV joint -- all the more reason to test on a perfectly smooth road or parking lot.
Sound and Vibration
A bad CV joint will generally exhibit a clicking or grinding noise as the bearings hammer together and the CV joint bounces around inside its housing. Along with that bouncing, the CV joint will exhibit a telltale vibration at various speeds and under various conditions. A clunking while engaging drive with an automatic transmission, a constant knock at low speed, and a vibration that increases with speed could indicate either a bad inner or outer CV joint. A humming or growling may indicate only a lack of grease in the inner or outer joint, but that will usually precede joint failure.
Inner vs. Outer Joint
You can think of the inner CV joint as the "power" joint, and the outer CV joint as the "turning" joint. Generally speaking, clunking, shuddering and vibration under acceleration or deceleration indicate a bad inner CV joint. Cyclic vibration -- vabration that regularly fades in and out under cruise -- may indicate the same. Outer CV joint failures will manifest as clicking and popping while turning -- you might even feel it in the wheel or floorboard as the joint constantly binds up and releases. Wheel bearings will often do the same, but often screech or growl while doing it.
The Hand Test
The simplest way to determine the condition of your CV joint is to grab the axle shaft, and try to twist it and shake it back and forth. A CV joint in good condition has very tight tolerances; you should percieve little to no movement while attempting to twist or shake the shaft. You might hear a barely audible click, but nothing more. The large bearing clearances in a worn or broken CV joint will allow it to flop around in the housing; the axle may twist if both the inner and outer joints are bad, or it may move by 1/8 of an inch or more. If you are not certain if the movement is within tolerance, check the CV axle on the other side as a frame of reference.