I think if you have a good relationship with a particular SA, they will do it. I know that my SA will happily do it, but I usually call her before I come in to ask if she has time. The answer is always yes, but I don't like to bring my second hand items unannounced, and I usually always do it during slow days (never weekend), just in case the item IS fake. I don't want a bunch of people staring. LOL.
Also, I never mention the real place I got it...i usually just say my mother gave it to me as a gift from a consignment store (don't mention any store names, etc.).
But like I said, I don't do this with anyone or at any store, just my fav SA because we have a good relationship.
I once walked in a LV store in a nearby city w/ a seller to authenticate a wallet and the SA's turned us down. They rudely told us that they do not authenticate and told us to leave, as we cannot do any second hand talking or buying in the store.
Hope this helps.