Are you taxed on retirement checks?

Yes, except for the portions of the checks that represent a return of after-tax employee contributions, qualified Roth IRA and Roth 401k distributions, and return of after-tax (non-deductible) traditional IRA contributions.

Note that withholding on periodic payments from pension plans is optional. But the fact that no tax is withheld does not mean that the payments are tax-free. You will have to calculate the taxes and pay them when you file your tax return at the end of the year. Note also that if you have a substantial underpayment of taxes, you may be subject to an underpayment penalty.

If you are referring to Social Security benefits, a portion of those can be taxable depending on your total income and your marital and filing status.