Step 1: Determine whether the work has been published.
FOR PUBLISHED WORKS
1. If it was published before 1924, it's in the public domain
2. If it was published between 1924 and 1989, the law required the copyright holder to process a formal application and to post an official copyright notice. Check to see if a copyright notice is posted on the item. If there is no notice, it's in the public domain. If there is a notice, contact the publisher for permission and post "In Copyright."
3. If the item was published after 1989, you must seek permission from the publisher to post the item. Depending on the licensing agreement the publisher made with the author, the copyright holder might be the publisher or the original creator may have retained their ability to license the item to others in that agreement. The best place to start in this case is with the publisher, as they are often easier to contact than an individual and they should have a licensing agreement on file.
FOR UNPUBLISHED WORKS
1. Copyright applies at the moment of creation and lasts for the term of the original creator's life plus 70 years after their death (it typically moves to their family). Seek permission from the current copyright holder, which may be the family of the original creator. Post only with permission from the copyright holder (the creator if he or she is alive or their family if they are not) and be sure to note that the work is "In Copyright."
Published works might include things like:
Unpublished works might include things like:
Always look for some kind of publication statement.