Good question that is! I dont know
The females tend to be very fat, and will be a lot smaller after they've laid there eggs. I guess thats the only way to see *shrug*
Read through this, its a good article on how to breed zebra danios, it might be helpful: Zebra Breeding
Mine tend to act differently. They REALLY flare at eachother and chase eachother more than usual. And when they come to spawning the male will rub at the female and push her around quite hard and then they twist around eachother and release eggs and munch them of course.
If the sexes are kept together, they spawn almost continuously, the females scattering a few eggs at a time. If you want a decent spawning, you should keep the sexes apart so the female has a chance to develop a good number of eggs.