It’s easy, if a guy smiles/makes cute gurgling noises at a baby, then either:
1. It’s his baby, i.e. he’s a daddy
2. It’s someone else’s baby, but other people’s babies are invisible to guys who aren’t daddies, so he must be a daddy
i.e.: he’s a daddy.
Q.E.D. 
Daniel said
ah, that explains why I’m not one who “smiles/makes cute gurgling noises at a baby”