Writing.Com General Discussion

if you can prove the song was published that long ago, you should be able to confirm the expired copyright/public domain status.

Also, look at the album that used it. Odds are good, since they didn't write it, they'll refer to it as "traditional" or "folk song" or something, rather than listing the album/band member's name for song-writing credit.

That lack of credit is also confirmation that it's in the public domain.