I'm an mot tester and we have a chart in the mot bay that has a list of every vehicle. It tells you what sequence the abs light has to come on "ie, light comes on and goes out after approximately 30 seconds" etc. if I bring a car in and there's no abs fitted or no light comes on then I look at that list and of the car and the year match one on the list that should have abs fitted then u go on the mot computer and fail it if the system is missing and fitted as standard or a problem with the light not coming on or the light not going off as it should etc.