how can u tell if its your period coming or if u might be pregnant

so you get white discharge throughout the cycle right? right. but  when you're sexually active and get the same white discharge as when your pregnant, after your fertile window is over (which means a couple days before your period), how can you know if your pregnant or if jts your period coming? like i know you could be pregnant and not know it (before u take a test), cause bc isnt 100% effective and if you dont feel any symptoms at all?