If the shock, strut, spring mounts to the lower control arm, the lower ball joint is under load. If the shock, strut, spring mounts to upper control arm the upper ball joint is under load.
Where the lower ball joint is under load, the vehicle is raised from under the lower control arm. This unloads the lower ball joint. If the upper ball joint is under load the vehicle is raised by the chassis so the suspension can hang. This unloads the upper ball joint.
Since almost all vehicles have struts with only lower ball joints those are the ones under load. So to check these the vehicle is lifted from under the lower control arm to unload the ball joint. Another way to check for a worn lower ball joint is, raise the vehicle until that tire is 5"-6" off the floor. Then take a 4'-5' long pry bar or 2X4 and place one end under the tire. While someone observes the ball joint, lift up on the tire and release repeatidly. If the ball joint is worn it'll be observed moving up and down in it's socket.