If you mean just by listening, I don't think a whole lot of people can. Only the very experienced. Most have to play notes to find the key. You're trying to match the notes played to the singer's voice without playing too many notes. If you can match the one, you're home free, usually. The one will sound decent with most of the notes of the song. A good practice is just to turn on you tube to a gospel list of songs and pick up the keys of the songs using your board. Also, there might be tutorials on YouTube.