A reliable fake news detector that my older relatives can use easily, and maybe more importantly, that they feel inclined to use instead of just believing and sharing every piece of information they receive.
It wouldn't be perfect but something that followed everything back to the original source by following links in articles and looked for loops might go a long way.
It already exists. If it's displayed on a television or the internet you can assume it is untrue. I generally accept only first-hand accounts and original sources for information.