You don't.
Some drugs or antibiotics can interfere with the way birth control pills work.
No, there's no way to tell whether a woman was taking birth control when she got pregnant (unless she is pregnant with the implant or IUD).
By using birth control or not have vaginal intercourse. If you already are pregnant (you formed the question a bit strange) the only way to terminate a pregnancy is by having an abortion at the doctor.
The best way to stop a girl from getting pregnant is to take steps to prevent the pregnancy in the first place by using birth control. There are lots of options: the depo-provera shot or birth control pill for her, condoms, spermicide, etc. Condoms are your best bet because they will protect her from pregnancy and protect you both from STDs
Yes you can. That is how most people get pregnant when they are on birth control. You should always use a second birth control method, like condoms, at all times any way
The same way you do. Condoms & birth control.
yes. no form of birth control is 100%. contraceptives(condoms) aren't 100% reliable. the best way to not get pregnant is to not have sex at all. or at least until you're ready.
No, 1/4 get pregnant that way.
Take a pregnancy test. That is the best way to know. Your body can give you fake periods when you are pregnant or you can have a hidden pregnancy where you don't have a bump. The best way to know is to take a pregnancy test...you can buy these or have one at your local doctors.
A tubal ligation is a highly effective method of preventing pregnancy. Birth control pills prevent, but don't help, a woman to get pregnant. Combining tubal ligation and birth control pills is not an effective way to get pregnant.
Birth control is not a 100% way to stop pregnancy and your period makes those chances even worse.