Most people draw on the examples of others in imagining the potential they see in themselves. They draw complex yet subliminal comparisons across similarities and differences, assuming that the achievements of those they consider similar to them are within their own reach, while assuming that the achievements of those they consider different to them are not. This is not wise. When you compare yourself to others, you operate within several layers of limitation in perception and understanding. What you think you see may not be as accurate as you believe, and what you do not see may be substantial. Furthermore, you are comparing who they are to who you could be, injecting even more sources of possible equivocation either in over- or underestimating your own potential.
Another method of estimating your potential is to extrapolate on what you’ve accomplished in your past. For those who have failed to accomplish much, extrapolation of past performance can provide a much needed wakeup call to vastly increase the intensity and consistency of your actions. But just as poor past performance doesn’t condemn you to poor future performance, neither does good past performance guarantee good future performance. Continuation of growth is required for continued growth.
Neither of these two methods are accurate enough for you to gain a solid understanding of who you could be. It is extremely foolish to assume that your potential is similar to another person simply because you know them, and it is equally as foolish to assume that you can know your capacity with any confidence before doing all you can to push yourself to your limits.