how do you know you have a flat spot on a tire or warped rotors?

 

Vibrations only while braking is the typcial symptom of warped rotors. There of course could be other things wrong, but that is the first to check. To really inspect it, you need to pull the wheels and get a dial gauge on the rotor faces.

If you hear a whump/thump that increases/decreases as speed increases/decreases...that might be a flat spot. You can probably just look at the tread and see anything major.