The purple area in that radar image implies there is range folding going on. The NEXRAD network radar product generators have algorithms for accounting for this, but they're not perfect. I am not 100% sure what it means if reflectivity is range folded but velocities are clean. Using spectrum width can be helpful, but it will not say "yes"/"no" on validity. SW only tells you the relative degree of certainty of the singular value in velocity. Remember: spectrum width is the standard deviation of the Doppler spectrum at that location.