Hi,
To do detect QRS complexes in EEG , the first thing you should do is to identify the most prominent complex of ECG. And thats " R -peak". Once you have identified the R peak , the rest would be easy , because Q and S do have some relation with R. If you have read the basic stuff regarding ECG then you would find it easy.
For eg. the voltage measurements of Q and S is less than R by some factor. Say Q is 1/4th of R something like that ( not actual, but you can figure it ).
So the first thing is to identify R. Since ECG is periodic, you can specify a window of a specific size ( depending on the sampling rate of the signal) to move between certain intervals and find the maximum value of voltage in a particular frame, which would be your R peak.
The other way would be using wavelets. You should have got an idea by now , I think so.