It's difficult to 'definitively' find a source. Best you can do is see if you can find paper sources attributing the song to a time that the copyright is definitely expired.

Asking the agencies is chancy at best. From experience, often they will tell you that the song is covered - when only one arrangement is. Many, many songs have had this done to them. Nothing wrong with it - arranging a traditional tune can be as creatively engaging as writing new material - but often there is confusion (which is to the publishing company's benefit) about the fact that while THAT arrangement may be covered, the actual song itself is quite firmly PD.

Of course, if you're not in the US, things are different depending on locale.

M