It's a smart idea, docs say. While there aren't significant studies on this topic, assume you're allergic until proven wrong, says Marsha Gordon, M.D., a St. Ives dermatologist in New York City. Check labels for your allergens. If the ingredients aren't clearly listed, apply a small amount to your inner arm twice daily for about 10 days before you use it on your face. That is how long it takes for some hypersensitivity reactions to occur. If any irritation develops, avoid the product.