  We propose a feasible approach to test Bell's inequality with the
experimentally-demonstrated circuit QED system, consisting of two
well-separated superconducting charge qubits (SCQs) dispersively coupled to a
common one-dimensional transmission line resonator (TLR). Our proposal is based
on the joint spectral measurements of the two SCQs, i.e., their quantum states
in the computational basis $\{|kl>,\,k,l=0,1\}$ can be measured by detecting
the transmission spectra of the driven TLR: each peak marks one of the
computational basis and its relative height corresponds to the probability
superposed. With these joint spectral measurements, the generated Bell states
of the two SCQs can be robustly confirmed without the standard tomographic
technique. Furthermore, the statistical nonlocal-correlations between these two
distant qubits can be directly read out by the joint spectral measurements, and
consequently the Bell's inequality can be tested by sequentially measuring the
relevant correlations related to the suitably-selected sets of the classical
local variables $\{\theta_j,\theta_j', j=1,2\}$. The experimental challenges of
our proposal are also analyzed.
