Read the credit line. Look for Public Domain – will not usually say P.D. (we do), but either the dates or the lack of a © symbol tell you it’s public domain.
If you have either (or both) CCLI or OneLicense.net – check their website for the song or the publisher. In CCLI the publisher will be the ‘catalog’.
Always give credit – your best policy is to include the credit line as it is on the piece of music. If you are covered under your license, put the license number: CCLI # 12345 OneLicense A-12345
If you don’t have a license to cover this song, you must seek permission from the copyright holder or whoever administers the copyright. If you have received permission, put the credit line and also “Used with permission” or “Reprinted with permission”. (see Sample Letter)