[ChrisLomont]

>Have you actually gauged 538’s own probabilities against themselves?

Yes, and the publish datasets of all their work to let others analyze them too. They're the most accurate of all the major sites.

Here's their github site [1]. It contains tons of datasets, past performance, analysis tools, everything you want to actually check if your claims are true or false, and most importantly, how accurate 538 has been over time.

Now try to do that analysis on, say, Fox News :)

Good luck.

[1] https://github.com/fivethirtyeight