Estimating the SNR of 2-FSK
I've used the first technique before. The idea is that you have some chunk of time domain samples where you know only noise is present and use that to get an estimate of the noise power, $P_n$. Then you determine the total received power, which will have contributions from both signal and noise, $P_{s+n}$. When you take the ratio of these, you get:
$$\frac{P_{s+n}}{P_n}=\frac{P_s}{P_n}+1$$
For high SNR, the extra $1$ doesn't matter too much but to be exact the SNR estimate should be:
$$\hat{\text{SNR}}=\frac{P_{s+n}}{P_n}-1$$