funniest thread ever
seriously though, go and rent or borrow another quality car (sorry but Jap won't cut it, unless it is a late model NSX)
Ring them up before you go in, make an inquiry about a lower spec Audi, ask them if they will do a trade in on whatever it is you have (hopefully it is something above $100k for an R8).
Make an appointment to test drive the lower spec car, show up in the "borrowed" car make sure they see you arrive and ask them to give you an evaluation for trade, then start asking about other cars, eventually working your way to the R8, ask if you can take it for a test drive, they will probably say no. Say you will be in touch regarding what you were able to test drive.
Come back a week later and ask again, make sure you get the same sales person.
You will find them to be more compliant this time around. This will be the time to start asking about what is required to purchase the R8.
Let them start to work out the figures, you will have to do your research here on how much you need to earn to pay for this. For gods sake do not say you work for someone else, even if you are a lawyer or Doctor. You will only be able to own this car if you have your own successful business.
Tell him you are really interested but you are also looking at a couple of other cars - Porsche GT3 or BMW M6 for example. However you will not proceed negotiations any further without a test drive. This may happen on the second visit or maybe the third.
You will not have to dress up (in fact some of the richest people I have met, wear shorts and T-shirts to this kind of stuff) under no circumstances act like a wanker unless you want them to bring the car to your $3+ million dollar Mansion for test driving a couple of days later.
But realistically I think they will call your bluff, if you have no history with them like Jamezilla said.
At that end of the market you are easily verifiable or you are a tyre kicker, simple as that.