We report direct measurements of the charging diagram of a nanoscale series double-dot system at low temperatures. Our device consists of four metal dots, with two of them in series forming a double-dot, and the other two serving as electrometers for the double-dot system. This configuration allows us to externally detect all possible charge transitions within a double-dot system. Specifically, we can detect charge redistribution in the double-dot, which corresponds to shift of an electron between two dots, using differential signal from the two electrometers. We discuss possible applications as an output stage for quantum-dot cellular automata architecture.