Greetings. PUVA therapy to diagnose vitiligo is not true. You may be looking for something called a Wood's lamp (a sort of blue light) that helps and assists in diagnosing a case of Vitiligo. Other instrument includes dermatoscope. Very rarely, even after clinical examination and usage of these tools it may still be difficult to be sure and then a skin biopsy becomes necessary. PUVA is a form of treatment used in Vitiligo.