I wouldn't be offended unless it stated "handmade" in the listing lol
I actually had someone ask me when I first opened if my products were handmade. I told them yes and immediately added that to my shop title and announcement.
Have you checked their shop about page? I don't use mine but a lot of shops do. Maybe their process is in there? But sure, just ask nicely. "Hey, I was just admiring your bags and I was wondering if you hand make them?" or something ...
But I'd check all their pages (policies, about page, profile, shop, listing, etc) first just to be sure ;)