I'm not a medium, but for me the best proof is when a person receives information they had no prior knowledge of from someone who passed. I have read stories of this happening and have even experienced it myself.

Here is a link to a newsletter you might find interesting.
http://newsletter.guidedafterlifecon...om/issue11.htm

It's all about people who are having guided afterlife connections with those who have passed. The section titled: "Evidence the Self-guided Afterlife Connections Are Real Connections" is especially fascinating because it gives examples of people receiving validations from deceased loved ones. These aren't just visions, dreams or tingly feelings, but real information that is later verified to be true.