Let's say a key signature has one flat, B flat. It could obviously be F major, but it could also be in its relative minor, D minor. I already realize you could tell the difference with the tonic, what chords it uses, etc. but none of those methods are foolproof.
So is there any way one can be absolutely, 100% certain a piece is in the major key rather than the minor key?
ETA: I understand there are similar questions already asked, but there has been no satisfactory answer (they were either too specific or no answer was given).