There are specific IRS rules for determining the taxable amount of a pension. 

Generally, most pension plan trustees will include both the gross distribution amount and the taxable distribution amount on a 1099-R and no additional calculations are needed to determine the taxable amount.

However, when the taxable amount is blank or when box 2b (taxable amount not determined) on the 1099-R is checked, the taxpayer’s investment in the contract must be determined and the taxable portion determined under one of the IRS specified methods. The method you must use is the method in effect the first year of the pension distribution. The method currently in effect is the “Simplified General Rule.”