Thanks for your question.
It is very simple she should do a pregnancy
test on the day of her missed period, If she has regular periods.
If she is not sure when her next period is due, she can do a pregnancy test 21 days (three weeks) after the unprotected sex.
A positive test result is almost certainly correct.
A negative result is less reliable. If you still think that she is pregnant, wait a week and repeat the test.
Hope this is helpful for you.