How do you check U-joints?

[HotRoddin]

Crawl under there and move it around ...shouldn't be any slack between in the u joint itself (movement between the bearing cups and the little shafts that go into them) also look for rusty looking stuff right at the edge of the bearing cups ... if its been a long time since they've been changed and you suspect them i'd just change em, they don't cost that much.
some of this stuff is hard to explain without visual aids !! lol