There isn't an easy way to do this but to cycle your suspension.
Get it up on a lift and let the axles hang. Make sure your brake lines, breather tube or anything else that can rip out is secured. Simply measure the distance from mounting point to mounting point.
Compressing the suspension isn't as easy. Try to articulate the suspension. Run it p a ramp or loading dock. By drive up the front end first, measure the most compressed side of the rear shock mounts (Good time to address bump stops too!). Back the rig up and do the same on the front measurements.
There's your extended length and collapsed length. It's not easy, but it's far more accurate than someone else's set-up on your rig.
