This invention relates to methods for diagnosing cancer, e.g., cancer of epithelial origin, by detecting the presence of tumor cells in a sample, based (at least in some embodiments) on the quantification of levels of four biomarkers, MUC1, EGFR, EpCAM, and HER2. In some embodiments, the methods are performed using diagnostic magnetic resonance (DMR), e.g., with a portable relaxometer or MR imager.