Run a carfax...
take the VIN to an Audi dealership and they will tell you what dealer it came from, if local, go by that dealer and ask for a report on the car if the other dealer cant do it. If it was serviced by them, then they will have all maintenance records for that car (if performed at that dealership).
My personal experince with 4ring is 50/50. I had a really good one and a really shitty one.... Audis are like VW when it comes to reliability and cost (Audis are for VW people who have gotten better paying jobs! And can afford to fix more expensive breaks....lol).