From my personal experience, vinyl (or "vinylite", whatever that is) was still a relative curiosity in the '40s, so 78s made of that usually said so on the label. Sometimes they were marked "non-breakable".
A record that's vinyl or some other non-shellac product will have a bit of flex to it, even the thick ones in the early days. Try to flex a shellac record and you'll have two or more pieces! And then there's Bakelite, which was an in-between product, neither shellac nor vinyl.
Playing any record with a steel needle raises the background noise level almost immediately. I began my record collection with 78s and 45s that came off of jukeboxes (my dad was in the coin machine business) and they were all very noisy.