How to find trim condition of a sectional airfoil without knowing the angle of attack?
If you have access to a model of your airfoil, a moment balance, and a wind tunnel, you may determine $C_{M_{ac}}$ experimentally: the moment the wing experience may be normalized to calculate $C_{M_{ac}}$  

Then solving for alpha is trivial. Otherwise a tool like XFLR5 may be useful to find  $C_{M_{ac}}$ for your airfoil.