How can i make sure it's actually spinning all four tires? On high and low, besides Gunning it.

 

Well the front differential is open so only one of the front wheels would spin.

 

Actually unless you have lockers, or limited slip differentials You will have one driven wheel on each axle. I suppose you could put the truck up on stands a check it that way but this isn't terribly safe.

 

on a rock driveway it should spin all 4, but the only good way to tell is to stomp on it on a (loose) rock driveway. then count the number of holes

 

This is only true if you have limited slip or locking differentials, otherwise only your right rear and right front will spin.

 

i wonder if he knows what he has, he might not know what diff he has. Ill look for that link to decode the door jam sticker

 

True. The front is almost guaranteed to be open. I found the codes for the rear, though:

The axle codes for F150s:
19 — 3.55 non-limited slip (F-150, Econoline)
17 — 3.31 non-limited slip (F150, Econoline)
H9 — 3.55 limited slip (F-150, F-250)
B6 — 3.73 limited slip (F-150, F-250)