How would you actually tax the ultra wealthy?

Assuming billionaires were going to get a special tax, how would you actually determine how much to tax them? Sure some would be straightforward like Musk where it’s entirely derived from a few companies with known ownership stakes, but what about all the others?

We don’t even know the names of most of the billionaires. With all the games they can play to hide money, now made even easier thanks to the changes Trump made in his first few months, how would you even figure out who and what amount to tax? They don’t have a normal salary or easily documented income like everyone else.