Main dealer won't check the car as well as a local independent garage to be honest, but they still check most things and take a reading of the ECU fault codes.
Checking the PCV is easy, have the engine at idle, when warm, then quickly remove the oil cap (only for a second, don't leave it off as it messes up compression). If the engine tries to stall, your PCV is good. You could always remove the dipstick (which again causes the same compression issues as removing the oil cap) and see if the engine struggles. If it idles flat the whole time, your PCV is faulty.
Checking the DV is a bit tricky if on a K04 (Edition 30/Pirelli Edition), and you'd need to put it up on ramps if it's a K03 (Standard GTI) as it's tucked right round the back of the engine. A simple way to test it's all functioning properly is to get a ride in a GTI that is stock and known to be fault free, then get a feel for it and if the one you look at feels sluggish or down on power, you can make a good guess as to why.
Check the service book and look at receipts. Just because a book has all the stamps doesn't mean everything's been done as it should, look at paper work for exactly what has been done. Finding out about recalls is a quick phone call to the local VW dealer who check a central database for you.
Other than that, you could check a car for a week and everything seem fine then it decides to throw it's toys out the pram one day a month later, perhaps never. It's a gamble, cars!
