I found this guy's video pretty informative.
The thing he pointed out is that you can visually inspect the rear axle and there is a mechanism that engages the locked differential. I would feel the most confident in that method because you are actually putting eyes on the vehicle to confirm the equipment (or lack thereof). The problem is that you need to know what you are looking for and where to find it.
The vindecoderz.com build list for my rig was eerily accurate down to the most minor detail. The one question is if something was modified or customized downstream. For instance, I was chatting with the local sales guy that imports a lot of Land Rovers from just across the border in Canada. He was saying that Richmond, BC has more millionaires and billionaires per capita than any other place in North America and that they will custom order the Land Rovers or order them and have them customized, so they get a lot of premium stuff that isn't entirely standard.
My problem with the gunning it uphill thing (which I tried) is that I wouldn't feel confident that I was doing it right or the system wasn't interpreting the conditions in such a way as to not command the rear axle to lock. For instance, I saw a video where a guy was showing an image of his 4x4 display as he was slogging through a foot of mud on a rutted dirt road and the center locker would flash briefly as orange locked and then go right back to showing green unlocked. To me, that really didn't confirm much, other than the fact that you can go through some pretty nasty stuff without locking differentials even factoring in.