It's against company policy for bags to be authenticated, but if you have a friendly relationship with an SA sometimes they may help. Also, I have heard that sometimes, some SA's may give it a quick look over, and state an opinion.
In Sydney Australia, only the manager or assistant manager is able to authenticate a bag, and ONLY if you have a Chanel receipt. I've had this done about three months ago.
Anita and Nicki from the US 1800-number say they certainly do authentications, but only if the bag was purchased from a Chanel boutique, and again, only if you have the original receipt (not a fax, nor a photocopy). this was from october 2006 so i don't know if company policy has changed since then.