Cross sections of $^{120}$Sn($alpha$,$alpha$)$^{120}$Sn elastic scattering have been extracted from the $alpha$ particle beam contamination of a recent $^{120}$Sn($^6$He,$^6$He)$^{120}$Sn experiment. Both reactions are analyzed using systematic double folding potentials in the real part and smoothly varying Woods-Saxon potentials in the imaginary part. The potential extracted from the $^{120}$Sn($^6$He,$^6$He)$^{120}$Sn data may be used as the basis for the construction of a simple global $^6$He optical potential. The comparison of the $^6$He and $alpha$ data shows that the halo nature of the $^6$He nucleus leads to a clear signature in the reflexion coefficients $eta_L$: the relevant angular momenta $L$ with $eta_L gg 0$ and $eta_L ll 1$ are shifted to larger $L$ with a broader distribution. This signature is not present in the $alpha$ scattering data and can thus be used as a new criterion for the definition of a halo nucleus.
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