SNR estimation prior to demodulation?
Perform a 2 dimensional cross correlation against a noise free replica of the signal you expect to receive with delay offset on one axis and frequency offset on the other, and find your maximum correlation. Normalize that to get $\rho$ and from $\rho$ you can get SNR. Do this after channel equalization, otherwise you will get the SNR degradation due to ISI as well. 

For further details, see: 

Noise detection

and

How can I find SNR, PEAQ, and ODG values by comparing two audios?

If the SNR is high and the noise is well behaved (thermal noise floor, no band selective interference present) then you can make a reasonable estimate directly from a spectrum plot such as the example I show below, where the SNR in this case was 40 dB as is clear from the distance between the signal and the noise. This is signal power to the total noise power that is in the signal bandwidth.