We present a new method, that is ideal for rapidly diagnosing the stellar magnetic fields from large spectropolarimetric surveys, like the near-infrared SPIRou Legacy Survey (SLS) at CFHT. Our new method based on Principal Component Analysis (PCA) is directly applied to the circularly polarised Stokes V line profiles and conveniently retrieves the main characteristics of the parent large-scale magnetic topologies, e.g., the relative strength of the poloidal and toroidal components and the degree of axisymmetry for the dominant field component and its complexity (dipolar or more complex). Further, our PCA method reveals the temporal variability of the large-scale magnetic field on various time scales.