On all driving school cars you will see a ‘certificate’ displayed on the windscreen..
A GREEN badge defines that a driving instructor is FULLY QUALIFIED and has passed all three DVSA qualifying examinations to become an ADI (Approved driving instructor). They are very tough examinations and only around 30% who start the training course actually qualify.
You will sometimes see a pink badge, this shows that the instructor is a TRAINEE and has not yet passed the final qualifying examination which tests the instructor’s teaching ability.