Firstly, he gives probability of outcomes. So for example, his predictions for the US election (http://projects.fivethirtyeight.com/2016-election-forecast/?ex_cid=rrpromo) is currently 82% Clinton vs 18% Trump.

Now, even if Trump wins, how do I know that it wasn't just the 18% of the time that he should've won.

The other problem is that his probabilities change over time. So on July 31st, it was almost a 50-50 between Trump and Clinton.

My question is, given that he has a different probability every day for the same future event with the same outcome, how can I measure how accurate he was for each day he made a prediction based on the information that was available up to that day?