I wasn't sure until about 3 weeks in when I could see them in her mouth
Although, when she stops eating and seems to be juggling something in her mouth, that seems to be a sure enough sign too.
I have observed some of my fish actually spawning but not result in the female carrying. I`ve only had my tank set up for less than a month and got one set of yellow lab fry, and a johanni carrying ! They appear to breed like rabbits if conditions are right!
I raised the PH and hubby freaked out (that was a couple of weeks ago) but it seems to have done the trick!!! We have several pairs and trios in the tank, and hope to see more holding soon. We have a lot of caves, but will add more artificial fry type plants soon. It was suggested in one posting that you even clip these fry "bushes" to the sides of the tank. Mama has taken up residence in a cave towards the left hand side of the tank, so we will concentrate these hiding places on that side. She definately isn't eating, although she looks like she'd like to.