tonality/ determination the key of the song/ harp played

I would rather say that very little is said about tonality which surprises me actually- I know the "4 ahead rule" but I want to ask- is there a way of determining the key of the song/ key of the harp played? can you do that by just hearing the song; I am not referring whether it is diatonic or chromatic or anything, just the key. As I have been observing the songs that are in second position in G are rather obvious- some of Little walters' famous songs.. But what about the others - there are so many different keys and I think people with no great musical background get a lot confused and checking the songs and comments on Youtube to find more about the tonality of the songs seems rather dull to me What do you think?