natural potential is not that hard to determine as far as overall muscle cross section potential is concerned. you can look at a number of things, including the lengths of your bones, muscles, and tendons to figure out just how massive you can become.
there are a number of other factors to be taken into consideration, like recovery ability (etc. etc.), but the basis of your potential can be figured very easily after a few months of training taking into account your insertion points (and so forth)