Doctors can diagnose an advanced case by looking at the skin. However, early or mild disease often requires a biopsy (removal and examination of a small sample of affected skin).
Because other diseases of the genitalia can look like Lichen Sclerosus, a biopsy is advised whenever the appearance of the skin is not typical of Lichen Sclerosus.
We highly recommend that you don't self-diagnose or self-treat. It's vital to have a doctor diagnose and treat your disease. However, be aware that biopsies for LS can be false negatives.
We might mention here, that a related condition, Lichen Planus, may also be diagnosed. It is similar to Lichen Sclerosus, but involves the internal vaginal canal, whereas Lichen Sclerosus only involves the outer vulva area.