They get tested against the standards that were in place, that's alright. They tax and/or disallow the whole standard. There are several different measures for

They get tested against the standards that were in place, that's alright. They tax and/or disallow the whole standard. There are several different measures for different things, e.g. a green/yellow/red plaquette with a number that straight up disallows old cars from accessing some central areas of cities. Then there's the Euro1-whatever rating on which the calculation of ecological tax is based.