  We construct an entanglement witness for many-qubit systems, based on
symmetric two-body correlations with two measurement settings. This witness is
able to detect the entanglement of some Dicke states for any number of
particles, and such detection exhibits some robustness against white noise and
thermal noise under the Lipkin-Meshkov-Glick Hamiltonian. In addition, it
detects the entanglement of spin-squeezed states, with a detection strength
that approaches the maximal value for sufficiently large numbers of particles.
As spin-squeezed states can be experimentally generated, the properties of the
witness with respect to these states may be amenable to experimental
investigation. Finally, we show that while the witness is unable to detect GHZ
states, it is instead able to detect superpositions of Dicke states with GHZ
states.
