What are some of the estimates that are used in determining whether underfunding has occurred or not?

Question:

What are some of the estimates that are used in determining whether underfunding has occurred or not? Do you believe that current disclosure requirements for pension accounting are adequate? Why or why not? You may use either and/or IFRS for your discussions on this topic?
GAAP
Generally Accepted Accounting Principles (GAAP) is the accounting standard adopted by the U.S. Securities and Exchange Commission (SEC). While the SEC previously stated that it intends to move from U.S. GAAP to the International Financial Reporting Standards (IFRS), the...