How do i know if the man has infection in his sperm?

STD? Other?: "Infection in semen [sperm]" is rare, unless you are speaking of sexually transmitted diseases. If he has been at risk for STD (other sex partners, unsafe sex, etc), the way to know is for him to be tested, and to be tested yourself if you've already had sex with him. And any rare non-STD infection won't hurt you. If no STD and if he has no symptoms, this is nothing to worry about.