I just want to add that if you are a pwcf and asthma/allergies (or some combination) if you let your asthma/allergies go it puts you at greater risk for exacerbations. so it's really in your best interest to be allergy/asthma tested so you can take care of it before it affects other things more. I am a severe asthmatic with severe allergies. In fact those two are often worse than the cf itself. I feel much better when allergies and asthma are under control. Right now they aren't and I've got the double whammy of a sinus/respiratory infection.
Like everyone else I've done all of the allergy tests. My dr relies on the skin tests. They rate your allergy level on a 1-4+++ (rating is based on skin reaction, 4+++ being the most severe). I am to some extent allergic to every food except for coffee and bananas (weird!) and airborne allergens except for aspergillus, kentucky grass, and ragweed. That means I'm allergic to approximately 120 things I was tested for. That sounds scary but avoidance of some things really helps me feel better, particularly the things that I test 3 and over to. Avoiding foods I was a 3 and above to made all the difference in digestion issues for me. Avoiding some of the airborne allergens (some are just impossible to avoid) meant I wasn't spending every other week in the hospital for ivs. We've done this testing for years and different/new things appear each time so it's been informative for me and helped me determine if what I'm dealing with is an exacerbation of cf or allergies/asthma so that we can treat it appropriately.
I think they can test for asthma with a methacholine test but I'm not sure they do this on cf patients, particularly ones with decreased pulmonary function. I know I have asthma because of my response around certain things and relief of symptoms through asthma medication. I do peak flow and it helps me gauge whether my symptoms are asthma or cf related. If it's asthma related my peak flow has a major fluctuation between blows (am is at my baseline and then by noon drops to 50% for example). With an exacerbation, the drop is more gradual over the course of several days, though hopefully I've caught it before it gets to the point that my peak flow has dropped 50%. I chart my peak flow because I've seen a correlation. I think this is unusual for cf patients with asthma to do.