How do a male know if he has low sperm count and he cums in the women and nothing happens?

Fertility Evaluation. That generally requires an evaluation. A semen analysis can be performed. Other specialized tests can be performed if needed by a fertility specialist to see what happens to sperm after sex.
Sperm evaluation. The only way to know about sperm count is to have it tested by a doctor. If you are concerned about this, talk with your doctor.