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Hard X-ray emission is generally interpreted as bremsstrahlung of 10-100 keV electron beams. This requires a high number of electrons to explain the intense X-ray flux observed. An interesting suggestion was made recently that 100 keV proton beams bombarding the atmosphere would create a hot thermal plasma at the origin of the observed X-ray emission. Such low-energy protons have never been detected. We propose to use impact linear polarization of chromospheric lines as a diagnostic of 100 keV protons. Recent observations of Ha linear polarization in solar flares are presented. A polarization degree as high as 2.5% is observed in some Ha-bright patches. The electric vector is radial. This polarization is interpreted as impact polarization by low-energy protons. From existing observations we cannot discriminate between local and nonlocal origins of the detected population of energetic particles. However, we show that nonlocal proton acceleration leading to chromospheric bombardment is a likely explanation of the observed polarization. Whatever the initial distribution, transport effects in the chromosphere generate an anisotropic velocity distribution for protons.