How do you know if section 199A dividends are from securities that didn't meet the required holding period?

[JohnW152]

Your best bet is to go through your records and hope you can find the relevant information.  You can't determine this from Form 1099-DIV itself, and it probably won't be helpful to contact the payer for the information.

In Box 5, Section 199A Dividends in the Instructions for Form 1099-DIV, the IRS instructs the Form 1099-DIV preparer:

Include REIT dividends (other than capital gain dividends and qualified dividends) for which it is impractical for the REIT to determine whether the recipient has met the holding period requirement described in Regulations section 1.199A-3(c)(2)(ii).

In other words, Box 5 can include amounts for which the payer has no knowledge relating to the holding period.