Very useful tip.
If you cannot remember which cars you have driven and you bought all of the cars, go to Grid World -> View Cars and select a car. Go to Sell on Ebay and there you can see how many mileage a car has. If a car has mileage on it, you can skip it and go to the next car.
If a car has 0 mileage, do a test drive with it. Drive at least a kilometer or mile and then finish your testdrive. Check Ebay again to make sure there is some mileage on them. Proceed until you have the achievement.
Note 1: There are more cars with 0 mileage then the number of cars you have not driven yet. This is due to the fact that you have driven some cars in the beginning of the game but you have bought them later on.
Note 2: You can buy 42 cars but there are 43 cars in the game. The last car is the Porsche 911 and you can drive that in Raceday (Also in World Tour but I couldn't find it). You can find it in the 24 hours of Le Mans series. I did 1 lap in it and then crashed it into a wall to complete the race.