Birth control and pregnancy

Okay so weird question but how does one know if they are pregnant while taking birth control? I know the placebo pills make your period come, so does that mean that it will still happen regardless of pregnancy or not? Or will you miss your period entirely regardless of the placebo pills? Ive always been curious of this, and if I were to get pregnant I would be high risk, so every month I have to take a test just to be sure.