The most straightforward test is to live it.

If you find that things work out for you and you are happy to continue with the way things are going, then it is a good indication that you have found your purpose. Furthermore, even if you haven't found your true purpose, wouldn't it still be good that you have found a way to live your life happily?

If you find that things are not working out for you, then you may not completely understand your true purpose, or you may have the "wrong purpose." In either case, the remedy would be to revisit the process that lead you to arrive at that purpose and revise your purpose using the new information you now have. I believe that it is purposeful to search for your true purpose. So, even if it takes you some time (or your entire life) to find it, you will be investing your time wisely.

For me, the process of one pursuing ones true purpose in life is about living consciously. This approach of conscious living is what will give life the most meaning. Don't focus your energy on whether you are finding the right purpose or whether you are finding your purpose the right way. Knowing and pursuing your purpose in life is meant to be liberating, not burdening. Enjoy the journey.