Howto calculate SNR for EEG data?
First you have to clearly define what you mean by "noise". Is simply the electronic noise in your measurement system or is it also brain activity that's not related to your stimulus. 

One easy way to determine SNR is to compare the signal level during a stimulus with the signal level without stimulus. If noise and signal are uncorrelated you can determine the signal level through energy subtraction and thus determine the SNR.  

Depending on your noise definition, this may not be practical cause you can't easily shut off the brain of the subject :-).