M dwarfs are the most common stars in the Galaxy and prime targets in the search for potentially habitable exoplanets. Their strong magnetic fields shape stellar atmospheres, drive winds, and critically influence the environments of orbiting planets. A reliable characterisation of these fields is therefore essential for both stellar astrophysics and planetary habitability studies. We investigate the magnetic field properties of M dwarfs using Zeeman broadening in high-resolution spectra. By comparing commonly used diagnostic techniques with synthetic spectra generated from magnetohydrodynamic simulations, we assess their ability to recover known magnetic field strengths. We show that some of the widely adopted methods can underestimate the total magnetic field by up to 50%, whereas another approach tested in this work provides a significantly more accurate recovery of the true field strength. These results establish quantitative benchmarks for the interpretation of M-dwarf magnetic measurements from intensity spectra and highlight important limitations of common diagnostic methodologies. Building on this, we are applying the best-performing magnetic diagnostic methods to a sample of M dwarfs, including prominent rocky exoplanet hosts, observed with the CRIRES+ near-infrared spectrograph at the ESO Very Large Telescope.