If you decide to necropsy, don't worry about fluid in the belly, just make note of it and pour it into a container. I do this to get an idea of how bad the ascites was. Once the fluid is out of the belly you can proceed with the necropsy.
I will attach a necropsy manual later today.
Look at the pictures in this thread too and they might give you an idea of what an ascites belly looks like and that might help with your needle placement.
I gotta make a comment on doing this... Please do it only if you are 100% sure that the swelling is just fluid. Is she breathing through her mouth? If not I might not wanna drain her.