United Family Hospital have an ENT department and can test for allergies, but if you're looking for a cheaper option then go to Tongren Hospital just outside the Chongwenmen subway station. It only costs a couple of kuai to "guahao" (take a number) and their allergy testing center and ENT department is one of the best in Beijing.

It can be a little confusing there if you don't speak Chinese so try to go with a Chinese friend if you choose to go to Tongren. After you've gone up to "take a number", they'll give you a card and ask you to put credit it on it to pay for extra treatments/medicines later. I recommend to put about 350 kuai on there. When you get to the allergy center, they'll swipe your card to charge for the allergy prick test. This costs around 200 kuai to do. They prock your arm about 20 times and drop a different liquid into each prick and get you to wait for 15 minutes. After 15 minutes they check if you've developed any hives, from which they can determine what you're allergic to.

After you've spoken to a doctor, he/she will then prescribe the relevant medicine which you pay for in the main reception area. Once you've paid, you have to queue up at the "gua hao" counter again to return your card and get refunded any remaining balance.