We only had the two A-bombs; Fat Man and Little Boy. Both were totally different designs. They weren’t sure if one, both or neither of them would work.
The prototype for Fat Man (plutonium bomb) was detonated at Almogordo, NM on July 16, 1945.
It worked, so it was a pretty sure thing that Fat Man would work also.
Little Boy (uranium bomb) made it’s debut over Hiroshima; that design was never tested previously. Obviously, it worked just fine.