there are aready good ways of measuring how good someone or something is at predicting things, for example Brier score
https://en.wikipedia.org/wiki/Brier_score

I have heard alot from rationalists about how accurate prediction markets will be/are and how one day soon everyone will use prediction markets to make every decision, I am wondering whether they actually are/will be impressive, I have also heard that manifold markets will not be as accurate since it doesn't use real money so I will give it some slack.

if someone calculates how accuarate manifold (or another prediction market is) with a sufficiently large sample size, in a way that they can convince me is a relatively random sample of markets, and where the people betting on the result couldn't influence the result to a large degree and with the predictions being analyzed are locked in sufficiently before the result happens (anyone can acurately predict something that already happened) and also finds or calculates similar metrics for individual people making similar predictions and shows me their analysis, then I will resolve based on how impressive I think the prediction markets results are.

I will resolve no if someone can show me that manifold markets (or another prediction market) is worse at predicting than experts. other than that resolution will be up to my own discretion of what i think is impressive.

I reserve the right to be impressed or unimpressed by anything.