How Can I Tell Whether Brake Vibration Comes From the Rotors or Tires?

My steering wheel shakes at certain speeds, but the vibration becomes much more noticeable when I brake. I’m trying to figure out whether I should be looking at the tires, wheels, or brake components before booking a repair.

Are there differences in when the shaking happens—only during braking, at a steady highway speed, or through the brake pedal—that can help narrow it down? I know a description can’t replace an inspection, but I’d like to understand what details are useful to report to a mechanic.