how do you know when your brake censor is bad

Take light cover off and check the socket or wiring with someone holding the brake pedal down and take a Multimeter and make sure there is battery voltage going to it if there is a bad ground you won't have battery voltage but if there is battery voltage it means the bulb is bad.

so if its a bad ground what do i do from there

I would do a continuity test on the ground to see if there is a short or open circuit otherwise you will have to have to go to a wiring schematic. I know guys who wired the High Mount Brake Light which is the third light for their toppers would splice into their driver side brake lights.