Try to find out if he retired before November 1996. Before that date, the Three-Year Rule applied (not taxable until contributions were recovered, as long as that happened in three years, which it always did, and then 100% taxable). But OPM said they would start calculating the annual exclusion under the new rules, either "General Rule" or "Simplified Rule," starting in 2000.
OPM said in 1999 that it planned "to calculate the tax-free amount of the annuity for each retiring employee and print this amount in a personalized booklet it will provide to each employee shortly after his or her retirement. This offers retirees a way to check their yearly taxable calculation because, although the taxable portion of an annuity changes from year to year with cost-of-living adjustments, the tax-free amount does not." The decedent may or may not have kept that information, and the widow may or may not know where to find it.