X-ray flares, lasting for similar to 100-1000 s in the X-ray band, are often observed following gamma-ray bursts (GRBs). The physical origin of X-ray flares is still unknown merely with the temporal/spectral information. On the other hand, some polarimeters are expected to be launched within several years, thanks to the increasing interest on astronomical X-ray polarimetry. Here, by assuming that X-ray flares are synchrotron radiation from relativistic spherical shells, we show that the linear polarization degree during the rising phase of an X-ray flare is much higher for the emitting region with toroidal magnetic fields than for that with random magnetic fields. In the decay phase of the flare, the evolution of the polarization degree is determined by the curvature effect of the emitting shell, which is a natural feature of jet scenarios for flares. Therefore, the measurement of the polarization of X-ray flares would provide a useful tool to probe the configuration of magnetic fields in the emission region, and may even help to test the curvature effect. The information on the magnetic configuration can further help us to understand the properties of GRB jets.