David, hill cane is smaller, with rounder stems above the nodes (terete). It's semi-deciduous to decidous, and has long, persistant node sheaths on the stems compared to the short ones on river cane that fall off the second year. There are also differences in the number of compressed basal nodes on the branches, hairiness of leaves, and configuration of air canals in the rhizomes. Basically, if you're in the southern Appalachians, and you see a short patch of cane growing on a hillside or ridge well away from water, it's likely hill cane (though it will also grow along streams.) If it's tall and growing in a floodplain, it's probably river cane. If it's winter time, and it doesn't have leaves on it, or only a few yellowish ones, it's probably hill cane. In the Coastal Plain, there is also switch cane, which is somewhere between the two in characteristics.
