  We present a scheme for a projective measurement of the parity operator
$P_z=\prod_{i=1}^N \sigma_z^{(i)}$ of $N$-qubits. Our protocol uses a single
ancillary qubit, or a probe qubit, and involves manipulations of the total spin
of the $N$ qubits without requiring individual addressing. We illustrate our
protocol in terms of an experimental implementation with atomic ions in a
two-zone linear Paul trap, and further discuss its extensions to several more
general cases.
