"Random test" means taking a production vehicle of any age, and testing it in real world conditions. Basically putting measurement devices over the exhaust and driving it on the highway, not in a lab.
It's less scientific but also so much more scientific if you expect the object you are measuring to be cheating in lab conditions - and they should be expecting that.
It's less scientific but also so much more scientific if you expect the object you are measuring to be cheating in lab conditions - and they should be expecting that.