The I.R.S. wants to know exactly how much your donation or bequest of art is worth if you claim at least $50,000, and turns to a little-known, secretive body called the Art Advisory Panel of the Commissioner of Internal Revenue for help putting an objective monetary value on it.

The panel, which consists of 17 prominent dealers, scholars, and curators from the Los Angeles County Museum of Art and the Toledo Museum of Art, usually meets twice a year to advise the I.R.S.