files manually installed in $prefix

[db]

I wrote a plugin file for a port and put it in the same directory as the default ones, because I considered it convenient at that time, not realising that it could be invoked by its full path from another location.

How can I get files manually installed in $prefix? I'm asking out of curiosity. I suppose I could get them by ownership if it wasn't modified or, less practically, by uninstalling installed and checking for remnants. Comparing recursive ls and port contents installed would be tricky. Any other ways?