Sonic tester measures thickness of roll bar tube. Say the spec on a particular bar is 0.125" or 0.083", for example.
If the tube thickness >= spec it passes. Thinner than spec it fails.
Inspectors also look for tube joints to be welded completely around (for example: at top of cage near headliner.) No grinding of welds. Gussets where required by rule book.
That's the basics. One of the members here is an NHRA tech inspector. Maybe he can fill in more details.
How fast is the car/ what certification are you looking for ? 8.50 to 9.99? 8.49 to 7.50? 7.49 and faster (1/4 mile times) There are new requirements to certify faster than 8.50 and 7.50 1/4 mile... ("funny car cage", SFI, etc.)