What is a good gauge of when to replace the U-Joints?

On some of my old cars you could actually hear them creaking. I imagine it is all a function of age and use. If there not broke don't fix them.  

A lot of times, just a visual isn't going to tell you much. but if you see a little reddish stain around the caps, they are failing. Vibration while downshifting can be a symptom. Sometimes you'll get a "pop" when going from forward to reverse.

If you have some doubt and they have some age on them, I'd go for a replacement. When a half shaft comes loose, it can really do some damage.

just did them all. 1/2 shafts and drive shaft. Had no idea how old they were, now I do. Will feel more comfortable on long hauls knowing things are new as compared to not knowing how old something is and how many miles are on it. It's called peace of mind. And the restoration just keeps going on.

[QUOTE=JimG] What is a good gauge of when to replace the U-Joints?[/QUOTE]

Just before they break and come thru the bottom of your floor!

You can usually hear a creak/click/squeak in them when they go south. The best way to check is to take the shaft loose, and twist the joint back & forth by hand. If there is any roughness/tightness to it, it is bad.