On the back of your Harmonica, there should be a letter. Most Harmonica's are made to play in the key of C, it is the most common key and the easiet key to play. If for some reason you can not find a key engraved into the back or the front plate of your Harmonica, the only other way to tell what key your Harmonica is in is to blow into the fourth key, now you will need either a piano or an instrument tuner, to test the tone, this will give you the key that the Harmonica is played in.