The Good Garage Scheme will mystery shop the 3000 garages in its network.
The initiative, which comes after a damning report from Which? about GGS garage standards, is designed to â€œidentify areas of training neededâ€.
GGS said the mystery shopper operation will be run by an independent specialist and will test reception, customer care, telephone call handling and the workshop.
Faults will be introduced to cars and a post-service inspection will check whether the problems have been identified and rectified.
The standard of workmanship and the way other garage staff interact with the customer will also be monitored.
Phil Dugmore, GGS technical manager, said the initiative is â€œan obvious and necessary development of the Good Garage scheme.â€
He added that where problems are uncovered, GGS will provide the garage with appropriate training.
The Good Garage Scheme works with the IMI, Delphi and Skillnet UK Motor Automotive Academy to provide training to its network.