A field-portable, single-shot and very simple method is presented for measuring the full complex degree of coherence (CDC) of a quasi-monochromatic Schell-model field using the Fresnel diffraction from a phase discontinuity. To validate the proposed technique, the CDC of the light emitted from an incoherent source with variable size is investigated. The results are in excellent agreement with theoretical predictions of the Van Cittert-Zernike theorem. This technique is also applicable to characterize the coherence properties of sources in other spectral regions, e.g., x-rays.