psy kids

how do psy know what ability their kids will have?

To a certain extent it seems to run in families but not always - Tanique Faiths half brother isn't an F-Psy despite the family nearly always producing them.
Nikita says the E-Psy ability which runs through her family often skips a generation but I don't believe that's true

They have some kind of prenatal test that shows what the dominant ability is. I can't recall which book it was in, but somewhere it said that Nikita knew Sascha was E-Psy while in utero and decided not to abort her because she understood that the PsyNet needed Es to function well.

In CbI it`s mentioned that there are in vitro psy test.