How Can I Verify The Cause For Urticaria?

How can I verify the cause for urticaria? Two days ago I ate lax with some unspecified see/food sauce and had a vicious hives on my left hip about 5-6 hours later. Yesterday I tried a little bit of crab in a see food market and 3-4 hours later the same hives appeared on my my back and other hip. I have not experienced any allergies before.