The instructors in my Fire Door Assembly Inspector (FDAI) class showed us a really handy gauge to measure clearances around fire rated doors, and since I'm going to do my first *official* inspection tomorrow I tracked one down.  It can easily and accurately measure 1/16", 1/8", 3/16", 1/4", and 3/8" gaps, and a 3/4" undercut.  For only $18.95, you can be the proud owner of your own Door Gap Gauge by shopping online at doorgapgauge.com.