try to start the engine from cold and use a bit of distraction techniques to buy time so that engine is left running for as long as possible like babbling about anything.
opt for a longer test drive.
prepare a check list and dont let them distract you with any finance deals.
When my DSG was misbehaving, it was absolutely fine when started from cold but if you had more than say 10 mins of stop-start traffic and the gearbox started to get hot, it was really obvious that something wasn't right.
Well just got back - 120 mile round trip, but it was worth it. Car was lovely, really good condition and low miles, one owner. Let it warm up for a good 5 mins before we went out. (thanks for the tip) drove really nicely, couldn't detect any obvious issues, and it has a 12mth seat warranty for piece of mind. Get it next weekend, can't wait!