you WILL see the ahc work
if its a sensor issue, it will sit visibly "odd". like its stink bugging or the opposite.
if its a bad ball diaphram, it will ride hard. it wont sit off kilter, but if you go over a wavy surface, it will jounce and rebound odd, as one hydraulic spring is essentially solid. little to no play.
if its the pump, something should be flashing on the dash, because the truck is trying to level itself and nothing is happening.
if its not working, and nothing is happening, i would suspect the system was shut off and fuses pulled. skulduggery. run. run fast. run far.
find one that is out of your price range but working to test drive and see how things are supposed to work, so you know what youre looking for.
never ask a dealership to fix an issue with a car on the lot. they have a net profit they want/need from that unit, and they arnt going to fix it the "right" way over the "cheap" way, knowing you havent a clue as to the difference. just walk away. the salesman isnt your friend. he represents the owners interest.