The Bios for the R5 cpu's is dated 10/4 (F6 revision) so I would imagine unless they have old stock that it should be fine by now.
For months I've been speccing those Giga boards, thinking well, any moment now the old stock will be gone. But after seeing old stock is still around even into August, I searched for boards manufactured after Ryzen 5 launch in April, with the right BIOS, and the Strix B350-F released on May 24th fits the bill.
I think they can tell from the serial number but I might be wrong on that. I agree that it is still best to ask the question first.
Yeah. It depends on whether the serial number is also on the outside of the box, as I know they can't open the box if sold as new (which is why they can't update it beforehand).