How do i find what's the cause of these hives?

Hard task: Assuming this is new for your, the most common cause is infection (often very minor in nature) resulting in activation of immune pathways in skin that trigger mast cells. Usually, hives resolve and we never know the cause. For other causes: http://emedicine.medscape.com/article/137362-overview#aw2aab6b2b3aa. Consultation with an Allergist will help