If you happened to ever let your PS+ expire, then the date you renewed it will reissue all your PS+ purchases on that date. So they will all show as purchased in your download list on that date. The only other way I know of checking is to have the game downloaded, click triangle, go to info and see if the game says it will expire. If it has an expiration date it’s obviously a PS+.
How do I know what PS3 games I own are PS+, and which I actually own?
BEFORE the nincomboobs coding the new PlayStation web store (while the old site was still around as default, or with the "psapp" backdoor), you could still use PSDLE to grab the ENTIRE purchase library (including themes, avatars, PS3, Vita, and PSP). Hopefully if you've ever run a PSDLE report and still have it lying around, that could isolate which content was given freely by PS+. Nowadays PSDLE only has access to your PS4 full title purchases on the "playstation.com" site, but there's still 1 other way to grab that list ...
In Chrome ... head to https://account.sony...mentnetwork.com , click on "Transactions", and either find the date you renewed your PS+ subscription, or navigate the dates until you find hundreds of transactions on the same date in 2020. Anything among those transactions upon that 1 date will be your PS+ free content.
Or if you're lazy, you might searching for emails from "PlayStation <email@example.com>" or "Sony Entertainment Network <Sony@email.sonyentertainmentnetwork.com>" and see what emails include the text "$0.00".