Do you live in PA? If you do, were the contributions to your 401(k) pre-tax? That is, were they added to Wages in box 16 of your W-2? (normally, 401(k) contributions are not deductible in PA). You would notice this if (1) PA wages are larger than federal wages (box 1) by the amount of the retirement contributions, or (2) if you call the HR department of your prior employer and ask them. If this is a PA employer, they should understand the question.
Any contributions that were not deducted in PA are considered basis for PA purposes, because you have already paid tax on them.
So add up all the contributions made to your 401(k) plan in those two years (or whatever) and enter this as the basis or "cost" of the pension.
