[Lil bastard]

The optimal way is to use a Dial Gauge on a fixed base or attached to the Caliper, though fixed and set against the rotor. Zero out the gauge and turn the wheel/rotor noting any variance in the gauge. I say optimal because a certain amount of runout (warpage) is to be expected and there is a tolerance - The permissible lateral runout for the installed brake disc is max. 0.06 mm.

Now, you can also check by taking an absolute straight edge and holding it against the rotor face at several points and look to see if the straight edge is touching the rotor for it's entire length, if not, you have some runout.

Another way is to take a board, drive a nail through it. Stand the board upright so the nail is touching the rotor and fix it firmly in place. Rotate as before and see if the nail comes off the surface of the rotor.

These last 2 will tell you if there is any runout, but not the value of it, and 0.06mm is a very small dimension, hard to determine w/o a gauge, though you could use a 0.06mm Feeler gauge and see if it will slip under the t=straight edge or the nail.

I have heard several reports that the aftermarket Zimmermans are prone to cracking and warping. They are not the same spec as the OEM ones supplied to Porsche. Probably a cheaper generic which is then drilled and tapped in the Porsche 5-bolt pattern (or any pattern really depending upon the application).