How to Determine if your Audi is Actually Short of Refrigerant

[Kneale Brownson]

Get a gauge set.
   
  You're probably not at maximum refrigerant charge, and your experience of poor cooling of the cabin on a hot day may mean your recirculation door is not working.

mboucher70 at hotmail.com wrote:
  Short of taking a car to an AC shop, what is a good method to determine if 
its actually short of refrigerant?

I was a bit surprised that the temperature got down as low as 42F. I had 
assumed that the system was low on refrigerant based on a very sunny hot day 
last summer with 5 people in the car. On that day, despite constant max/AC 
and max fan, the car just never got very cool.