How do I know if I itemized deductions or took the standard when I look at previous tax returns?

You can look at line # 40 on Form 1040.  If you have any other form (1040A or EZ), you took the standard deduction.

If you Itemized Deductions you will have a schedule A in your return.  If you can’t tell then tell us the amount on 1040 line 40.  If you filed a 1040EZ or 1040A short form you took the Standard Deduction .

There is no Line 40 on the 2018 1040.  The 2018 Standard Deduction flows from 1040 Worksheet, Ln. 8, to the 1040, Ln. 8