If they are arranged side-by-side, the one on the left is generally HOT. If they are stacked, it's anybody's guess. Easy way to find out: turn on your water heater, let it warm up, attached a hose running to the tub/shower or sink (or into a bucket) to one of the valves and turn it on. It may take a little time for the cold water to purge from the line, but you'll know for sure. Just my 2c.
2013 Eagle 266RKS
2011 Ford F-150 w/3.5L Ecoboost & H.D. Tow Package