[bcsco]

How do they know it’s not just the estate of the local rich man?

Because he couldn't do it on his own. Someone else had to have helped him...

[JoeProBono]

[Sacajaweau]

I read that they found the government loan papers.

Of course, everyone knows that Fremont was more than a little political so we'll skip that.

Anyways, there is likely diary after diary of his adventure and map after map and someone finally nailed it.