Is there anyway of seeing how much memory an individual domain is using?
Unfortunately no. You could only do that on your local development server.
However, if it begins using too much memory on our servers, you can expect to here from us 
If it is windows, I can tell
I have magic tools 
If the domain in question is on windows, and you want to know, just send me a PM with the domain and server, and I can tell you after monitoring it for about 30min to an hour.