If you have access to your filed 2015 federal return, the easiest way to determine whether you itemized your deductions is whether you filed Schedule A, Itemized Deductions with your return - also, the total from Schedule A would be reported on LIne 40 of your 2015 Form 1040.
If you filed your 2015 return with TurboTax, you can obtain a copy of it to verify - please see the following TurboTax FAQ, "How do I get a copy of a return I filed in TurboTax Online?":
https://ttlc.intuit.com/replies/3301525