Car shudders when brakes applied

No. You need a special gauge, called a dial indicator, that mounts to the brakes and has a needle that scrapes along the rotor surface. You zero it when the needle is just touching the rotor. Then spin the rotor slowly. The gauge reading will fluctuate if the rotor surface is not perfectly smooth, this is called runout. If the runout as shown on the gauge ever exceeds the maximum allowable amount (a Haynes manual will have these specs) then the disc can be considered warped and will have to be machined or replaced.

With rotors getting thinner and cheaper, it makes more sense to just replace, rather than machine.

If you don’t want to buy the dial indicator, then take it to a shop. They’ll test them for you.

-Matt