I don't know if it fits your definition of easy but take a look at this post/thread for a script I wrote. Detecting user installed pkgs - #8 by spence
It just uses what I think are the basic installed utilities but can take several seconds to over a minute on low powered processors.
It works fast and correctly on my NanoPi R4S.
EDIT: A bit slower on my tp-link EAP225 and Netgear GS308T devices.
Be sure to sanity check it against a manual package listing before trusting the output.
Also have a copy of the list of packages saved to your PC (off the OpenWrt device) along with config backups etc before upgrading which is good advice for any device all the time.
I hope it helps!