Yes, the IRA/SEP/SIMPLE box being marked indicates that the distribution is from a traditional IRA.
Forms 1099-R for regular distributions from traditional IRAs are required to have box 2a equal to box 1 and box 2b Taxable amount not determined marked because the IRA custodian has no way to know whether you've made nondeductible contributions to traditional IRAs or not. It's your responsibility to report your contribution either as a deduction on Form 1040 or Form 1040A or as a nondeductible contribution on Form 8606. To see your basis in nondeductible traditional IRA contributions, you would need to look at your most recently filed Form 8606. If you've never filed a Form 8606, presumably have no basis in nondeductible traditional IRA contributions (unless your IRAs acquired basis from after-tax money from a qualified retirement plan rolled over to a traditional IRA or by the transfer of basis to you with the IRA from a divorced or deceased spouse). Chances are, if you don't already know about any basis in your traditional IRAs, you have none.
