Generally, the issuer of the 1099-R will have an amount listed in Box 2a for the taxable amount.
If you see a (zero) in Box 2a of your 1099-R, the entire distribution amount in Box 1 is nontaxable.
If this is a Roth Distribution that has been held for 5 or more years and you are withdrawing the contributions only, report zero for Box 2a.
If you made nondeductible contributions to a Traditional IRA and are withdrawing only the contributions and not any earnings, report zero for box 2a.
If you had a direct rollover from a qualified plan to another qualified plan, report zero for box 2a.
If none of these apply, and Box 2a is (blank), the taxable portion of your distribution needs to be calculated. If you begin receiving annuity payments from a qualified retirement plan, you may use the Simplified Method to figure the tax-free part of the payments. Under the Simplified Method, you figure the taxable and tax-free parts of your annuity payments by completing the Simplified Method Worksheet in the Form 1040 Instructions.