Generally speaking, retail stores will not officially authenticate bags. It actually puts them in a position of potential liability. I asked related questions about a Fendi spy (spoke to SAs and customer service reps of Fendi and NM).
NM customer service reps indicated that they guarantee any bag sold by NM is authentic. They (corporate level or store level) do not "authenticate" bags. They indicated that a buyer would need to go through the designer for authentication.
Fendi indicated that authentication would not happen at the store level. If you wanted a Fendi authenticated, the bag would need to be sent to Italy to be inspected by (someone, someplace...I don't remember exact details)...serial numbers would be traced there, etc.
I received virtually the same information about a Hermes bag.
There are individuals that are considered somewhat expert in the authentication process that may offer related services (and may charge a fee). For example, you purchase or are thinking about purchasing a Balenciaga bag...neither Barneys nor Balenciaga NY (store) "officially" authenticates bags, but would could send an email to dudeIloveyou, pay a small fee, and s/he will give you her opinion regarding authenticity.
The only "opinion" you can get at the store level is "well it looks good or not"...again, the stores don't want to provide you with an opinion that would put them in a difficult position... I wouldn't even take my bag to the store unless I had a good relationship with a SA there.