Was it "Flow Simulation" where you were shown this?
If the bottle is fully closed, then Flow Simulation has a Check Geometry Tool which will calculate the internal fluid volume.
Try this link
I dont know if you can get more accurate than mass properties in solidworks?
Simply create a new (non merged) body to completely cover the existing body. Then combine-(subtract) the two bodies. This should leave two bodies (the outer shell and your internal core) delete the outer body and you can then use mass properties to calculate the volume of the core.