I doubt if there is any way to find poorly merited quality posts among someone's post history in a quick way.
You can bookmark
[self-moderated] Report unmerited good posts to Merit Source and take some reported unmerited posts by forum members[/url]. It saves your time but sure not all good unmerited posts are reported there.
Or in
The future of Bitcointalk: Low Ranking Top Merit earners in the past 30 daysI am thinking of another way, if you want to aim at a specific member. Check their merit distribution over boards/ sub-boards with Ninjastic.space; get their most merited boards; check their posts in that board in which the user earned most of merit.
Example:
Probably it is one of methods scan for potential good posts.