If you play better and sound more clearer on a 3, then that's how you can tell.

Finding a reed strength is not the type of thing where you continuously upgrade over the years. Although many teachers have their students start on a 2 or 2.5 and then hopefully upgrade to a three after 2-3 years, it isn't a good idea to upgrade until you can find a reed that will play with your embochoure comfortably on your instrument with your mouthpiece and ligature. I know players who start on a two and keep moving up and up every few months. It's stupid!!! Many professionals (maybe even most) find that a 3 to 3.5 Vandoren is good enough to provide a good tone for them. I know a high school player who is already on a 4.5! Take your time, practice, experiment, and find what's best for you. Good luck!