Recover frequencies from IQ samples
You should take the FFT on Is+j*Qs. If you have say 1000 IQ samples, you can form 1000 I+jQ complex values and take 1000 point FFT to get another set of 1000 complex numbers. If you plot magnitude of these output values, it will show you the frequency content of those 1000 IQ samples.

Since you took 1000 point FFT, each of those complex output values correspond to frequencies at multiples of $\frac{F_s}{1000}\text{Hz}$ where $F_s$ is the Sampling Frequency. For example, the first value will be corresponding to frequency at $0\text{Hz}$, next value is the frequency content at $\frac{F_s}{1000}\text{Hz}$, 3rd value corresponds to frequency $\frac{2F_s}{1000}\text{Hz}$ and so on..

If you are using numpy fft, the documentation should be a good enough guide to you.