How do I know if my handbag is a knock-off?

I got it as a gift from my sister. I know (as a matter of fact) that her mother-in-law frequently buys her designer bags and other accessories, which she can afford to do. I think she may have re-gifted the handbag to me. I don’t want to ask and embarrass or offend, but I’d like to sell the handbag and need to know if it’s real or not. It’s just not my thing. What should I look for to determine authenticity? It’s a small handheld by BCBG (allegedly).