WARNING: This page is about a different car, the 2005 Audi A4, 2004 Audi A4, 2003 Audi A4, and 2002 Audi A4. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Checking Synchronizer Ring For Wear
Courtesy of VOLKSWAGEN UNITED STATES, INC.
Press synchro-ring onto reverse idler gear cone and measure gap "a" with feeler gauge.