How to check the FFT results of a sine wave?
You need to plot the magnitude of the output of the FFT.  I'm not familiar with your programming language, but in Python you would use something like plot(abs(fft(a))).  For a silent input, the output should be all zeros.  For a sine wave input, you should see two spikes:



For a real signal, the spikes will be symmetrical from left to right.  If you're doing a real FFT, though (which is more computationally efficient) you'll only get the left half of the plot as your output, since it ignores the redundant mirror image.

If the frequency is higher, the spikes will be closer to the center.  If the frequency is perfectly in sync with the chunk size, the spike will only be one point wide and everything else will be exactly 0.  Otherwise it will have a tapering "skirt" like above.