Dielectrophoresis is widely used for cell characterization, and the exerted force on cells depends on the difference of polarizability between the latter and the surrounding medium. This physical phenomenon is translated by the real part of the Clausius-Mossotti factor. It is mostly modeled from the imaginary part, measured by electrorotation. The method described here measures experimentally the real part of the Clausius-Mossotti factor. It relies on the cell velocity when submitted to pure dielectrophoresis, and it was conducted on several human cell lines, at different times. A variety of cell lines was evaluated, from different organs or representative of different stages of cancer, with promising findings for early cancer detection.