First, you have to buy them dinner to get them in the mood. Then, later, offer them some wine. Then some more wine. The rest should happen naturally.
From
http://www.planetinverts.com/Red Cherry Shrimp.html :
Sexing
Sexing the Red Cherry Shrimp is very easy. Females are easy to identify as they are larger than the males, have a much darker red coloration, and also have a curved underbelly. In the photo below you can see how the male is smaller, has very little red coloration and the "under belly" is a straight line with no curved shape. After keeping the Red Cherry Shrimp for a little amount of time you will be able to sex this species without a problem.
Male and Female Red Cherry Shrimp next to each other:

Females will also have a "saddle" which is the common name for eggs which are still in the ovaries, located behind the head on the top of the bottom. It is called a Saddle due to the close appearance to the saddle you would find on a horse. Most "saddles" are yellow in color, however they can also be green. "Saddles" are a sign of sexual maturity as well as soon to come eggs. The theory on why the saddle is green is based on the fact that the wild Neocardina Heteropoda has a green saddle. It is thought that the green saddle re-emerges due to the wild type genetics.
The eggs of the Red Cherry Shrimp are mostly yellow but can be green as well. If the saddle is green then the eggs will be green and vice versa. There is no difference between having yellow eggs or green eggs. One is not better than the other and it is also not a health indicator. Please see the photos below for the different colorations of saddles and eggs on the Red Cherry Shrimp. A photo of this shrimp with yellow eggs can be found at the top of the page.
Female with yellow Saddle

Female with green Saddle

Female with green eggs
