What label is it? US Decca, UK Brunswick, etc? And what's the catalogue number? I might be able to find out if it's particularly rare or possibly a bootleg.

As for being able to tell if it's the same song, the B-side should be Thirteen Women which ran nearly a full minute longer than Clock (Clock ran 2:08 originally, while Women is something like 2:57). I know it's not quite so easy to do this with 78s as it is with 45s but you might be able to visually tell if one side has a noticeably longer groove than the other.