A method for the analysis of temperature and field dependent magnetization data near the Curie temperature TC is devised, which allows the reliable determination of critical exponents even in strongly inhomogeneous ferromagnets. It is shown, both experimentally and theoretically, that the field-induced broadening of the ferromagnetic phase transition follows a power law with a characteristic critical exponent β⋅δ, where β and δ are the conventional temperature and field dependent critical exponents of the magnetization. Experimental results on inhomogeneous La2/3Ca1/3MnO3 films yield critical exponents β=0.3682±0.0033 and δ=4.76±0.32, even though the intrinsic TC distribution width is larger than the field-induced effect.