If you have a ramp to use, then that is a good way, or try this, or use a pair of floor jacks at opposite corners of the truck. Even if you can't max out the suspension, you can get a good idea on shock travel by measuring at rest, then compress one corner as far as you can, measure the change in shock length and spring travel, then extrapolate to where the bumpstops would limit up-travel. Down-travel is easier, HiLift the tires off the ground for full droop and use a small bottle jack to force one side down. I run a double shackle in back (still have yet to unbolt the upper part) and have plenty of shock travel with angled 9012s in back. I made my upper shock mounts adjustable so I can fine tune the shock angle for the amount of travel, if needed.