If the concertina matches THIS TONE it is a Key of 'C' concertina. (5 press A)
If the concertina matches THIS TONE it is a Key of 'B Flat' concertina. (5 press G)
If the concertina matches THIS TONE it is a Key of 'E Flat' concertina. (5 press C)
If the concertina matches THIS TONE it is a Key of 'A Flat' concertina. (5 press F)
If the concertina matches THIS TONE it is a Key of 'D Flat' concertina. (5 press B Flat)
Most concertinas will be close or preferably it will match exactly to one of the above tones.