How do you know when a U Joint is bad

[Ford7.3IDI]

Hello all
Im not really sure where I should post this but my truck is making a loud clunking noise when im driving seems to only be at speeds below 35 mph and i was told it may be my drive shaft U joints? any help would be appreciated

[sethc24]

Just get under the truck and grab and wiggle your driveshafts to check the u-joints for play.

[PowerstrokeAaron]

sometimes they will look like they dont have play when they do try using a pry bar if u need to.

[gblauf]

Broken U joints make the most noise under power. It your hearing during acceleration, thats probably it. They make a clickity clack sound if you wanna get technical! If your only hearing 1 big clunk, then i suspect, broken spring, pad or loose ubolts, etc.