The cover of Coral Magazines premier issue has a fry being born, with it just coming out of it's fathers pouch. A very impressive photo that I have never been able to catch!
A pregnant seahorse will have a very pronounced pouch, and will be able to swim without problems. He will actually have a very round abdomen, even when his brood is small. Just before delivery, that is. Until he starts to "show", I do not think there is any way to tell if he is carrying.