Comment by wongarsu

Search for public records about the image. If you can confirm the author is dead for at least 70 years (in case the author originally held the copyright) and the work is at least 120 years old or was published more than 95 years ago (in case this was work for hire) you are good

For younger works other conditions might make a work public domain, like the work being created by a US federal government employee as part of their duties