Heres a link I found if you still have questions feel free to ask as I am sure others **cough ** cough will chime in. lol

Here is a link that might be useful: Calendula, Marigolds

Pot Marigolds(Calendula) are different than regular Marigolds(Tagetes.) On regular marigolds, the flower does not drop off. It sort of withers away. Once the remnants of what was once a flower becomes completely dry, you can pick them and open them up. Mixed in with the flower remnants(chaff) will be the long thin black seeds. Ones that are really thin and bendy are not good.
If you are worried that the flowers are not dry enough, just pick one as a tester first before removing more. You'll get to know by sight after awhile, and it will be second nature.
Remy