How do I calculate the taxable income on my 1099R. This is the a percentage of my ex's pension that I get per my divorce decree (I am not retired, he is).

I receive a monthly payment from my former spouse's pension. He is over 60 and retired - I am 48 and not retired. The 1099R I received has "Unknown" as the Taxable amount. I have not withheld any taxes from these payments, although $150 of my monthly disbursement is applied to my survivor benefit. I am not sure how much of the amount is taxable - and/or if I need to pay a penalty.