Securely chock the front tires, then jack the back end up so the rear tires are off of the ground. Place it on jackstands under the axle.
Crawl under the truck and you can start with either the front or rear U-joint. Use two hands and try to twist it back and forth, up and down and see if there is any play in the joint. I'm not describing the technique very well, but it should be apparant if you manipulate it in opposing ways if there is any play in the joint.