Find the note that feels like home: the resolution point of melody and bass. The key signature or final chord usually confirms it.
Hum along until you find the one note where the song feels finished, at rest, home. That note is almost always the key.
Shortcut: songs very often end on their home chord, and the last bass note is a strong clue.
By ear: establish where the music resolves. Sing the note that feels like a full stop; that is the tonic. Then judge major versus minor by the overall color.
From chords: list the chords and find the set they all fit; the chord that opens sections, ends phrases and feels like arrival is home. From notation: the key signature narrows it to a major key and its relative minor; the ending usually decides which.
Beware two traps: songs that start away from home (many begin on IV or vi), and songs that change key partway.
The rigorous version is scale-degree hearing: once your ear locks the tonic, every note becomes a numbered degree with a felt function, and the key is simply where degree 1 lives. Functional ear training (cadence first, then judgments) builds precisely this reflex.
Ambiguous cases are real: modal songs (Mixolydian rock reads as major with a lowered 7th), relative-key hovering (is it C major or A minor: check whether phrases resolve to C or to A), and modulating bridges. Name the section, not just the song.
The trainer’s Find the Tonic and key-context modes drill the two halves: feeling home, and judging distances from it.