I am not sure that it was one particular experience that made me realize that I had the ability to be a medium. The first time I felt a presence around me was about three weeks following my grandmother's death (no it was not her), I was in my office and I felt a man standing behind me. This experience was not startling at all, there was such peace and an incredible sense of wonder about it all. That was 1996 and that same presence is still with me, his name; Grey Owl(my guide). You have to keep in mind that I never believed in this 'stuff' before, I did not go searching for it, and I certainly wasn't looking for attention... it happened as naturally as anything we do in life.
No my grandmother's death had triggered something deep within me and I felt a huge change coming. Please realize that everything about my life changed... not just what I was able to hear or see. Back in 1996 I weighed over 250lbs and today I am 170lbs (pictures on this site show that), I changed from the inside out, completely and spiritually.
As for that one defining moment, I honestly didn't have one (that I can recall). It took years of messages for other people in order for me to truly come to the conclusion that I am a medium. The great thing about what I do is that others have to lable you a medium (not myself), since they are the ones validating the messages. I could tell you I am an astronaut, or a pipe fitter or anything.... but what others say I am is a Medium. Thank God... what a wonderful way to help people. It is all about trust... trust in myself, trust in God.
If I asked you to look back on your life for one defining moment, I would suspect it would be hard for you to find just one. Life is grand in that one event leads to another and it is only when you look at the whole picture do you see the beauty that we call LIFE. Thank you for a question that caused me to walk down memory lane, and I hope I answered your question.