Many a song has some variants in lyrics and tune, and many are changed in the run of time.
Mostly you can only a give a "terminus post quem" - the time when it was first recorded, which can be very early, e. g. medieval manuscripts or renaissance printings.
When you are lucky you can find a reference to a special song in an early source before the time of its recording, maybe only a short quote or a line.
Historical references to persons or events could be misleading; e. g. a German song about the French war of 1870/71 (Napoleon I.) could stem from a song about the French defeat of 1812/13 (Napoleon I.) with some changes to the then modern times.
So the good tidings are: You'll never know for sure, and you have the possibilities of endless learned disourses (the phologue's delight).