Learning to pick out the key to a song (the “tonic”) is a very useful skill to have as an “ear” musician.
Many of you have found this tip very helpful…
Simply go to your favorite genre and start listening to the samples.
Just take your time and listen for that ONE common note that can almost be sang over any chord in the song. Try to fast-forward yourself to the end of the song and figure out what the keynote of that chord would be. Often times, the keynote (root note) of the first chord is the key of the song… but not always.
Also, you can review this post for more tips on finding the key.
Have fun with this one!