I've asked myself the same question and one always wonders where Forbes gets it data and how accurate it is.
Think of what is publically available.
1. Rule 144 filings if ownership of a public company is involved.
2. Property tax records and in most states the filings of trusts that may hold the property.
3. If the government asses the tax they have 1099s
4. Public auctions
5. Plane, boat and car registrations.
6. Litigation discovery (this actually shocked me when I heard it was a wealth source data point)
7. ERISA Filings
Still a lot of noise in the data - but where is most of the wealth in the US.
Stocks, bonds, real estate, jewelry, and art.
For the moment I'll assume rare cars, firearms and watches fall into jewelry and art
There have been a handful of public complaints against Forbes, but I think you'll find they net out over/under.
Plus if you pass a tax you create a reporting requirement.
Does regulatory arbitrage then occur - do you scoot your wealth out?