Nothing on your Form 8889 directly indicates that you used the last-month rule. You used the last-month rule if you contributed more than pro-rated amount you would have been eligible to contribute without using the last-month rule.
For example, if you covered by a self-only HDHP plan for only the last 9 months of 2020 and you were under age 55, your maximum contribution for 2020 without using the last-month rule would have been 9/12 of $3,550 = $2,662.50. However, being HSA-contribution-eligible for December 2020 allowed you use the last-month rule to contribute a full annual $3,550 which is more than $2,662.50.
