How do I know if I paid the underpayment penalty?

[DoninGA]

The penalty will be included in the federal taxes owed or it will reduce the federal tax refund.

[VolvoGirl]

If you have a penalty it will be on 1040 line 38.  It is included in the tax due on line 37 or reduced a refund on line 35a.  Nothing you have to do.  

[Mike9241]

the penalty if any will be on line 38 of the 1040.