Figuring Out Key Signature

[MasterProX]

Uh, what key signature is this song in?  Also, is there any easy way to determine what key signature a song is in?

[Olimar12345]

I hear A as tonic.

[DonValentino]

I hear A as tonic.

Agreed, I'm pretty sure it's A minor.

EDIT: Tried a part on the piano and yes, it's A minor.