After you have determined that you have a Chemnitzer-style concertina (not an English, Irish, Anglo-German, Duet, or Bandoneon), you should determine what "key" the instrument is in.
The Czech Area Concertina Club have a convenient decoding sheet: see How to Determine Your Concertina Key and How to Play with Concertinas in Other Keys.
If you have a 52-button or "104-key" (which really means 104 notes - 2 for each button), you should have a button on the right side that looks like a circle with a crosshairs through it (a "coda" symbol). The note that sounds while you draw (that means pull the bellows open) while pressing this button is the "key" of the instrument.
