How can I find out if the timing belt has been changed on a used Audi Allroad with over 100000 miles on it?

You can call an Audi dealership and they can look it up IF it was done at a dealership. Otherwise you can pull the inspection cover and check the belt condition or look for signs of the water pump being newer....but there's no definitive way to know unfortunately without a receipt or record from Audi.