Some shotguns have smallish firing pins that do not need bushing, the Greener Empire shotguns I have owned have all been fine, including the 'Fat Lady'.

Older guns, and especially hammer-guns with angled firing pins, mostly have over-sized pins and holes, and definitely need bushing (or avoiding!).

I had a couple of comparative photos up on this site years ago, they will still be here somewhere but no idea how to find them now. I will have a look tho.....

In short, there is probably no way to tell if an unknown gun, photographed closed, is bushed or not, or has small enough holes not to need it. You will have to request photos of the breech face (standing breech) if possible.

Good luck!

OK, found this already uploaded: shows Greener Empire standing breech with firing pins small enough to not need bushing: