You should receive Form 1099-R, Distributions from Pensions, Annuities, Retirement or Profit-Sharing Plans, IRAs, Insurance Contracts, etc.. The total amount of the payments and distributions you received during the year is reported in box 1, and the amount taxable as ordinary income is shown in box 2a. These amounts are reported on lines 16a and 16b on Form 1040, or on lines 12a and 12b if you use Form 1040A.
If your Form 1099-R does not show the taxable amount, (the box should be checked) you will have to use the General Rule explained in IRS Publication 939 to figure the taxable part you need to enter on line 16b for Form 1040, or 12b for Form 1040A.