VCDS does show the mileage on screen when connected and it does always differ slightly by a few miles, so this may be taking the reading from the ECU rather that the digital display?
i think this is a reflection on the fact that the ECU stores in km, and calculates it into miles. i was reading a thread on another forum that said that you can't check, because the way they change it is by changing the settings on the ECU. it isn't stored in other places - places like the gearbox take the reading from the ECU.
whether this is true, or not, or whether you can use VCDS, or not, there is no replacement for your senses and basic checks like checking the MOT and service history. but EVEN THEN a member of another forum bought an S3 from an Audi
main dealer (who i assume did an HPI check etc) and checked the MOT documentation when he got online to discover the car had been clocked. if the car looks older than it is and you're unsure then walk away.