1. Listening to the Song: An experienced musician or singer can often identify the original key of a song by ear. Certain notes and chords sound more "home" or stable in a particular key.
2. Sheet Music: if you have access to the original sheet music for the song, it will usually specify the key signature at the beginning. The key signature indicates the key of the song.
3. Key of Instruments: Certain instruments, such as guitars or keyboards, are often tuned to specific keys. If you know the tuning of the instrument used in the original recording, it can give you clues about the song's key.
4. Music Theory: If you have some knowledge of music theory, you can analyze the song's chords and harmonic structure to determine the key center. The key center is usually the chord that sounds the most stable and provides resolution.
5. Reference Recordings: Sometimes, listening to the original recording of the song or another well-known version can help you identify the key. The recording may include musical cues or hints that reveal the key.
6. Online Resources: There are also various online tools and resources that can help you analyze a song's key. Some software can automatically detect the key of a song.
Understanding the song's original key can be helpful for transposing it to a different key, playing along with the music on an instrument, or analyzing its music composition.