Easy for them to tell

1. Lack of a strong distinctive heat source under the car where your cat (normally the hottest part of the exhaust) is meant to be (they have a device that can detect this I believe)

2. When a car is 'catless' the smell of the exhaust is quite distinctive because it is a much more pungent smell due to the cat not being there to do it's job.

$10,000 fine last time I checked