When collecting thangka paintings, one should know that works by higher lamas and spiritual leaders are generally considered more valuable. However, as thangka painters very rarely used the medium for personal expression, the scrolls were rarely attributed or signed, except in very rare cases. Many old thangka paintings feature inscriptions on the back with a mantra about the deity depicted, but it is rarely a personal note from the artist. That said, signatures can often be found on thangkas produced for personal meditation.

（Reference：https://www.invaluable.com/blog/thangka-tibetan-buddhism/）