Can you share a couple museums that have good examples of using alt text?
Many large museums have good examples of using alt text, and some of them have been covered by the media or have written their own guides on how to use alternative text.
For example, Art in America, an illustrated monthly magazine focusing on American Art, uses the Museum of Contemporary Art in Chicago and the Metropolitan Museum of Art in New York as an example of alternative text done well. It includes some details of how the MAC in Chicago and the MOMA in New York both use alternative text, including guidelines for the descriptions used in alternative texts. Link here: How Museums Are Making Artwork Accessible to Blind People Online.
As community member Heather mentioned, The Cooper Hewitt Smithsonian Design Museum also has a very detailed guide for alternative text that contains some of the museum’s own examples. This guide is located here: Cooper Hewitt Guidelines for Image Description.
Finally, WebAIM has a great guide for creating alternative text with their own examples here: Alternative Text.