I think it's hard to tell just by looking at someone. Obviously, you can evaluate structure and muscle attachments, but it's the response to training, and later to hormones, that will be a good clue as to your potential. Assuming diet and training are on point, and I don't mean scientifically optimal, just that you are getting the required nutrients and doing just the basic training most start with, you will get a good idea after the first year. That age can be anywhere from your early teens to twenties. If you start bbing in your 30s I don't think you can be on the Olympia stage as you have past your prime years to achieve your full potential.