Measure the SNR of a signal in the frequency domain?
It can be a little tricky.  If your sine wave happens to fall at an FFT bin center, things are a little easier.  If your sine wave happens to not fall at a bin center, you have to consider spectral leakage.  You'll also need to consider how your window function affects your signal.  

A simple technique to estimate the signal power would be to sum the squared magnitudes of the FFT bins with most of the signal's energy (i.e. the largest bins).  To get the noise energy, sum the squared magnitude of all of the other bins.  This will get you close.