so to introduce a “wealth tax” would mean to create a new category (“Federal Wealth Tax”) that taxes all net wealth except real estate (which is already taxed on the state level), am i seeing this correctly?

The details haven’t been worked out. It may or not include locally taxed properties. California’s one time billionaire tax I don’t think cares if it’s been taxed or not already.

how do you assess the total wealth of a person, once they have moved their stuff abroad?

They can’t move a lot of it. It’s mostly shares in companies, bonds, commodities, all traded on public markets. Some is in private markets. But that’s still all managed by banks. Banks all around the world started sharing data in 2016. So governments now have access to people’s international holdings.

Things like cars, boats, paintings, various high value sundries, they have insurance values that could be used.

In short, very carefully. Some things may be missed, but the vast majority of their wealth can be found and calculated.