How do I know if I itemized deductions in 2017?

By looking at your 2017 tax return.  If you filed a 1040A or 1040EZ form then you used the standard deduction as itemized deductions were not allowed on those returns.  If you filed a 1040 form for 2017, then look at the amount on line 40.  If the amount on line 40 was $6,350 for single or MFS, $12,700 for a joint return and $9,350 for Head of Household then your 2017 return used the standard deduction (plus there would be additions if you were over 65 and/or blind).