If you are getting payments from a retirement plan, and Box 2 is blank, it can be a headache to figure out the taxable portion.
You will have non-taxable benefits only if you made after-tax contributions to the plan. Because plans differ among states and school districts, there is no way to tell whether a plan has any after-tax contributions without contacting the HR department of your employer, and even they may not know.
If boxes 5 and 9b are both blank, and you didn't make after-tax contributions, then the full amount will be taxable. Enter the 1099-R exactly as written. When TurboTax asks about the taxable amount, give it your best answer.
See this link for the TurboTax FAQ on entering taxable pension income.