There are two ways to tell,

1) watch your population count as they kiss.

2) you can do details on the mother before she goes in the hut it'll tell you if shes nursing or not.

or 3) or interupt her from going in the hut and see if shes says she nursing and a baby pops into her arms..

or 4) keep trying to breed her and if it says shes already nursing...

ok so I lied there are 4 ways...