How do you know if your alergic?

Get allergy tested. Allergy can present with many symptoms including: skin rashes (eczema or hives) runny or stuffy nose (heyfever or sinusitis) or trouble breathing or coughing (asthma). If these symptoms intefere with your life then going to an allergist to get tested and developing a plan to deal with the allergies and their symptoms.
Allergy testing. You can suspect allergy if you get sneezing, runny nose and itchy nose or throat. However, the only way to tell for sure, or to determine to what you are allergic is to have allergy testing with a board certified allergist.