I'd ask whether it's had a DSG oil change, for starters.
Then I'd ask about its general oil and coolant usage.
If I'd known, prior to ordering, that my A3 2.0Tdi would go through a litre of oil every 2000 miles, I'd have thought twice.
Coolant usage is a whole different ballgame though, potentially requiring a new cylinder head.
If you're buying from a garage, I would ask them for the previous owner's details and call them.
I've done it before, no-one's ever minded the call.
If someone were to call me about my 2.0Tdi DSG Sportback, I wouldn't necessarily warn them off it, but I would be truthful and leave them to make their own decision.
There are loads of the things around, so don't force yourself to take anything that worries you.