  We propose an approach to nondestructively detect $N$ qubits by measuring the
transmissions of a dispersively-coupled cavity. By taking into account all the
cavity-qubits quantum correlations (i.e., beyond the usual
coarse-grained/mean-field approximations), it is revealed that for an unknown
normalized $N$-qubit state $|\psi_N>=\sum_{k=0}^{2^N-1}\beta_k|k>_N$, each
detected peak in the cavity transmitted spectra marks one of the basis states
$|k>_N$ and the relative height of such a peak is related to the corresponding
superposed-probability $|\beta_k|^2$. Our results are able to unambiguously
account for the intriguing multi-peak structures of the spectra observed in a
very recent circuit-quantum-electrodynamics experiment [Phys. Rev. A {\bf 81},
062325 (2010)] with two superconducting qubits.
