WARNING: This page is about a different car, the 2009 Audi S8, 2008 Audi S8, and 2007 Audi S8. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Checking Cylinder Head For Distortion
Courtesy of VOLKSWAGEN UNITED STATES, INC.
Check cylinder head at multiple points for distortion, using straight edge and feeler gauges.