Microresonators offer an attractive combination of high quality factors and small optical mode volume. They have emerged as a unique platform for the study of fundamental physics and for applications ranging from exquisite sensors to miniature optical combs. Characterizing the linear and nonlinear properties of a microresonator is the first step toward new applications. Here, we present a novel in situ method to measure the nonlinear refractive index and absorption coefficient in microresonators. Laser-scanned transmission spectra are fitted by a comprehensive theoretical model that includes the thermo-optic effect, Kerr effect, and back-coupling of counter-propagating modes. The effectiveness of our technique is demonstrated by evaluating the nonlinear indices and optical absorption of silica and chalcogenide (As_2S_3) microspheres at 1.55 μm. Significantly, our method also quantifies important parameters including the quality factor, thermal relaxation time, and back-coupling coefficient at the same time. Our findings provide a powerful new approach for characterization of microresonators and optical materials and pave the way for new opportunities in the area.