Finding the key means locating the harmonic center of gravity of a song. Some notes and chords create tension while another chord feels like arrival or rest. That sense of home is one of the strongest clues to the tonic.
Start with the chords
Write down the chords that appear repeatedly. In tonal music, many often come from one scale. A progression containing C, F, G and Am strongly suggests C major, although context still matters.
Listen for resolution
Pay attention to the end of phrases, choruses and the song itself. Dominant motion often creates an expectation of resolution. G moving to C is a classic clue that supports C as tonal center.
Use melody and bass
Long melody notes and bass resolutions can reinforce the hypothesis. If the vocal line settles on C while the harmony also resolves to C, the evidence becomes stronger.
Major or relative minor?
Keys such as C major and A minor share the same basic note collection. Listen for which chord truly feels like home rather than relying only on the scale notes.
Watch for modulation
A song can change key. An automatic detector may report the predominant center, but a detailed chord sheet should mark the new key where the modulation actually begins.
A quick confirmation test
After choosing a likely key, play its tonic over places where the song feels settled and then test the dominant chord. If dominant-to-tonic motion feels natural at phrase endings, you have strong supporting evidence.
Do not force a single answer when the music is modal or changes tonal center. In that repertoire, identifying a center for each section can be more useful than assigning one global key to the entire recording.