We propose and demonstrate a coherent population trapping (CPT) scheme, in which a CPT dark state is constructively and destructively detected by a bi-chromatic light field with proper polarization and timing setting. Electromagnetically induced transparency (EIT) and electromagnetically induced absorption (EIA) resonances are simultaneously observed in a vapor cell. A differential-CPT (diff-CPT) signal, with doubled amplitude and suppression of common-mode noise of light and of the atomic ensemble, is then obtained by subtracting EIT and EIA resonances. This approach might be of great interest for the development of high-performance atomic clocks, sensors, or high-resolution spectroscopy.