Im no expert but I just brought my second DSG Mk5 at the weekend and there were a few things I looked out for:
1) No clunk when you put it form N to D just a gentle sensation letting you know this has happened.
2) It can find Reverse easily and quickly.
3) When going up through the gears you (almost) cannot feel the change only hear it.
4) When idling the car does not stall.
The gear change should be like butter, nice and smooth and almost unnoticeable. If the car has any funny moments on the above situations then cal my walk away but the best thing for you to do would be to drive a few and get a feeling for them.
Good Luck!