Front - Remove springs (be it coils or t-bars or coilovers) droop out the suspension fully then measuring at the hub (bearing cap, whatever) raise the suspension to its fully bumped position. The distance traveled from bump to droop as measured at the hub will be your suspension travel.
Rear - With leaves, jack your truck up so the tires are off the ground, measure from the top of the axle tube to the frame, or wherever your bumps bottom out. That is the amount of possible travel you have.
We Jeep guys and any solid axle guys won't have more travel than our shocks measure in the front, it just won't happen unless you lean your shocks either way in or way back. IFS guys (beams, a-arms) have more travel because of shock angle and placement.