They determine the liability using Generally Accepted Accounting Principles (GAAP). Under GAAP, you take all of the estimated liabilities, based on reasonable actuarial estimates of life expectancies, medical costs, etc. and then discount them (opposite of compounding) back to a present value using a reasonable rate of interest. You then calculate the estimated tax receipts assuming a reasonable rate of economic growth and then discount those amounts back to a present value using a reasonable rate of interest. The liability is the difference between the two numbers.
Bottom Line: It's a SWAG (Scientific Wild-Assed Guess) that is subject to incredible amounts of manipulation by economists, bureaucrats and others. Hence the debate between the GAO's estimate of a 70 trillion debt (increasing at 5-7 trillion per year by the way), and Professor Kotlikoff's estimate of 211 trillion, and others who put the amount north of 250 trillion.
About the only thing that you can say for sure is that there is no way in hell that this country can ever fulfill more than a meager portion of its obligations. If you are younger than age 55 and count on any meaningful amount of retirement benefits, health care benefits, veterans benefits, housing subsidies, or pension benefits from the government, then you will be very disappointed.
I understand all that, but over what TIME PERIOD? SS is not due to close out so how can anyone claim a total value for it since it is perpetual?