box 2b is not checked on form 1099-r
is disribution in box 1 taxable?
Impossible to answer without more information.
What code is in box 7?
Is anything in box 2a?
Is the IRA/SEP/SIMPLE box checked.
If box 2b Taxable amount not determined is not marked, the amount in box 2a is the taxable amount that the IRS will expect to see on your tax return.