You can’t test for UPF at home, but you can rely on UPF ratings, since UPF-rated clothing sold in the U.S. has already been certified. However, keep in mind that if the shirt gets stretched out or wet, it may lose some of its UV-blocking capabilities. If you want to find out if an unlabeled t-shirt you have at home will provide any UV protection, try holding it up to the light. If it’s sheer, it won’t provide adequate sun protection.