Head/body/mouth shape between the two is easy once you've handled enough of each but the easiest way to tell between Greaters and Silvers is the tail (caudal) fin.  Silvers will typically have a grayish tail fin, Greaters will have a redish or orangish tail fin.  There are Greaters in the Chippewa river though Silvers are significantly more common.  River Redhorse are not uncommon to see on some parts of the Chippewa, which is a protected species in WI and that could be confused with Greaters - both being large, redtailed redhorse.