I've participated in a couple of Spec Miata impounds over the past season. Checking restrictor plates on 1.8L cars is easy, as is verifying the diameter of the brake rotors.

This particular SM has a 47mm restrictor plate as required for 1994-1995 Spec Miatas. Other sizes that might be seen are 45mm (1996-97) and 41mm (1999-2005). The plate may be checked with dividers through the throttle body, but removing it entirely takes only a wrench and a phillips screwdriver. The plates should have a very slightly undersized opening, but there was a QC problem at MazdaSpeed and some oversized plates were delivered to unsuspecting racers. Any SM racers reading this would be well advised to double check the dimensions of their plates.

This is the left rear brake rotor from a 1.6L Miata. Typical SM wheels are very open (trying to achieve that 13lb minimum weight for a wheel), so it's easy to check the rotors. Since 1.6L rotors are about 1" smaller than 1.8L rotors, you almost don't need a tape measure; it's really obvious what you're looking at using the good old Mark I Eyeball tool. Some drivers suspect 1.6L cars of using the larger later rotors for better braking; other drivers suspect later cars of using the smaller early rotors to get less mass attached to the wheels. I have yet to actually come across one of the rumored cheater cars.