the main difference are visible on the front of the car , the impala has four square lights with the turn signal light just under , while on the Caprice the turn signal lights are on the bumper
the front lights on the Impala are a little higher than on the Caprice , and there is a visible difference on the back , where the backing lights on the caprice surrounds the License plate , while on the impala , the backing lights are into the rear lights
the hood & fenders are just the same
But this is only applicable to 1977-1979 Chevy's , don't ask me more on later models , it would become intricate
the pic's below come from a 1977 catalogue , the Impala is the red car , the Caprice being the blue one...