There are some tools which "automagically" do what we're talking about here, but the process is always the same. It's only a question of "does it require much user intervention to make it happen?"
When I need to know this, from within Pro/E (or any similar solid modeler) I do a solid volume and then perform boolean "cutout" operations. Sometimes additional manual "trimming" is required once all that is done. The resultant solid can then be analyzed.
If you're doing a fluid-fill-level you can create a parametrically-driven cut which "slices away" the "oil volume" to represent air. If you're smart, you can set this up so that your entered value is "volume of fluid" (or even mass of fluid if you prefer) rather than "fluid level" and the "fluid level" is then your output.
But there is no way, within a tool such as Pro/E, to calculate a volume unless the volume you're evaluating is modeled. All modeling withini Pro/E and similar programs is solids-based. So you have to have a solid volume to analyze, and this needs to be a separate component.
Using computational-fluid-dynamics, there are some pretty nice auto-volume-generation tools. These still work in very much the same fashion as above, however, but are much more automated. I believe all CFD packages have this functionality because you can't do a proper CFD analysis without knowing this. But the process, ultimately, is the same as what we're talking about here.
It's my hope that, eventually, PTC will write an additional module of code which will do an automatic "volume fill" feature in an assembly. You'd have to set a threshhold gap value, of course, or you'd have to create "boundary surfaces" along open gaps (vent holes, etc) before this would work, of course. It doesn't seem at all difficult, and it would save a lot of time for the end users.
But for now... you really have two choices - copy surfaces and solidify, or create a volume and do boolean subtraction of volumes. And no matter what, you're creating a new part to do it with.
