"We have very, very bad information on the income of high-income people," Hamermesh says.
There are a lot of public records and ways to find out what somebody is worth, but unless you have access to somebody's bank account you can't know it all.
Forbes says it goes through Securities and Exchange Commission filings, court records and calls analysts, employees, competitors and even ex-wives.
"We do not pretend to know everything on a private balance sheet," the magazine states when explaining its methodology.
Forbes says it tries to include everything, from stakes in publicly traded or privately held companies, to real estate, art, yachts and mansions.
But in the end, it's an educated guess. When Forbes says somebody is worth $7 billion, it means that the person is worth at least $7 billion.
So what are some ways that the rich can shield their assets?
Michael J. Zuccarini, a partner with Smith & Zuccarini, a law firm near Seattle that does tax, business and estate planning, says that one of the best ways is to set up a revocable living trust.
Putting assets in such a trust doesn't always shield them completely, but makes them harder to find.
Zuccarini says that many trusts are set up in somebody's initials. So, for instance, if you were searching for Bill Gates in a computer database of land records he wouldn't show up because the property might be under the name of the B.G. 2007 Trust.
Zuccarini says the biggest disclosures often don't come until a wealthy person dies.
Most states require a will to be filed in court, even if it doesn't go through the probate process. Additionally, some states require a detailed inventory of assets.
The wills of many celebrities have been made public this way, and some can be found on the Internet.
Court TV has an entire section of its Web site dedicated to the wills of famous dead people, including Jacqueline Kennedy Onassis, Babe Ruth, Richard Nixon, Elvis Presley, John Lennon, Dorris Duke, Jerry Garcia and Marilyn Monroe.
But putting assets into a trust can preserve that financial privacy, even after death.
Another vehicle some rich people use to shield their assets from scrutiny is to set up a limited liability company.
Zuccarini says that confidentiality is one small reason to set up a trust. Many of his clients do it for a bevy of other reasons.
But those who do create trusts however, don't want others to know that they have a trust. So while all of somebody's money might be in the trust, Zuccarini says his clients don't want the trust's name on their checkbook.
"I do have a quite a few clients who don't want their neighbors to know how much money they have," he says. "One of the major reasons: They want people to change their view of them. Many live modestly."
There is one more reason.
"People are afraid of being sued," Zuccarini says. "The more people that know they are worth a lot of money, it's like running around with a target on their chest."