They're educated to check for the compliance plate/decal, that must be fitted and visible if it is to be legal. That is a very simple check.
Any doubt and they can call for a vehicle examiner to attend. Sadly there aren't many qualified VEs about any more.
Ok - I thought they would be
but if that "education" is anything like some of the "training sessions" I had as a teacher then it would be a 30 minute talk after normal work on a wet cold Tuesday evening
and we were expected to remember and stick to it 5 years later
clearly they can easily check for a throttle I suppose but it would require a QE (?????) to check whether or not it only took the bike to 6 mph or was a full throttle
Motor cut off speed would need a proper check
all in all - would a copper on duty want to spend the time and energy checking it out for a minor offence
of course - if it is a moped that could open up other offences such as helmet and reg number stuff which could make it worthwhile
sorry - just woffling