It depends. If your 1099-R has a box 7 code of 1, this is a distribution subject to penalty. Thus, if you have a code 1 in box 7 of your 1099-R, you do need to determine the amount of pension you received before you turned 59 1/2 and after turning 59 1/2. (If, on the other hand, you have another code in the box, such as a 2, your distribution is not subject to penalty. It is subject to income tax regardless of the time of the distribution).The amount you received before turning 59 1/2 is subject to penalty, unless an exception applies. (You would have an exception, for example, if you started taking your pension if you were separated from work. This exception applies to anyone 55 years of age and older).
As far as splitting the amounts is concerned, you may need to contact your plan administrator if you are unsure of the amounts. You can, however, enter the amount you received on the Exception to Penalty Screen as shown in the screenshot below:
{edited 1/28/18: 4:46 PST}
More of a question...How are you receiving it?
DID you take it all out at once and roll into an IRA?,
...or are you getting monthly payments (annuitized)?
IF getting paid monthly the issuer would/should give you two 1099-R forms, one with a box 7 value of "2" and one with a box 7 value of "7". The one with the "2" is before 59.5, and the "7" is after 59.5.
(though, I admit I'm not certain all follow that procedure)
Thank you @SteamTrain for the clarification. I have edited my response to include this information.