How do i know i contributed to HSA based on last month rule?

[ghost]

For the prior tax year (2015 in this case), did you contribute an amount that was greater than the amount determined by prorating for the actual number of months of HDHP medical insurance coverage?

[Opus 17]

The question refers to 2015 (last year).  Generally, you can contribute $3350 per year to a single coverage HSA.  If you only had qualifying HDHP insurance for part of the year, you can only contribute part of the maximum. (Like, you can contribute 4/12th of the maximum if you had HDHP coverage for 4 months.)

The last month rule says that if you had qualifying HDHP insurance for all of the last month of 2015 (your qualifying insurance started on or before 12/1), and you keep the coverage for all of, then you can contribute the full maximum for 2015 even though you might not have had qualifying insurance for all 12 months. 

But, if you made a maximum contribution in 2015 using the last month rule, and then you stopped having qualifying HDHP plan in 2016, your 2015 contribution becomes retroactively disqualified and an adjustment has to be made on your 2016 tax return.