Not usually.

For boys, there is no medical way to know if they are sexually active unless they have an STD.

For girls, someone would only know if they were sexually active if they were pregnant, had a baby in the past, had an STD, or had sperm in their vagina.

Other than that, the body does not change after sex. The genitals will still look the same.

Not really no. The easiest way would finding an STD, but even then some STD's can be given to the child at birth.

what are you afraid of?