there are online sources that will tell you which Walmarts
Many RV websites show information that is out of date.
Unless it was an official list on the Walmart website, or a list that is officially updated regularly.
I personally wouldn't trust it.
I have found listings of campgrounds online, and on arrival found the no longer exist.
The link above talks about the large decrease in the percentage of Walmarts that allow camping. I would hate to drive up, stay overnight and discover it wasn't allowed.
After all, the whole marketing of the idea is they want you to go in the store and buy things.
If you pull in the lot, and can't see signs.... A 10 second discussion with the greeter before a purchase, would remove any questions.. "yes sir. You can camp, but only in the NW corner of the lot". Or "no sir a new bylaw was passed a week ago that prohibits it" or "we have had a bunch of robberies in the lot and we no longer allow overnight parking"