Subversion currently doesn't track where something was branched to, only where something was branched from... so there is no feature that can tell you which of all locations miss a certain 'merge'.
For a specific branch you can see if it still needs specific merges via
svn mergeinfo --show-revs=eligible
$ svn help mergeinfo
mergeinfo: Display merge-related information.
1. mergeinfo SOURCE[@REV] [TARGET[@REV]]
2. mergeinfo --show-revs=WHICH SOURCE[@REV] [TARGET[@REV]]
1) Summarize the history of merging between SOURCE and TARGET. The graph shows, from left to right:
the youngest common ancestor of the branches;
the latest full merge in either direction, and thus the common base
that will be used for the next complete merge;
the repository path and revision number of the tip of each branch.
2) Print the revision numbers on SOURCE that have been merged to TARGET (with --show-revs=merged), or that have not been merged to TARGET (with --show-revs=eligible). Print only revisions in which there was at least one change in SOURCE.
If --revision (-r) is provided, filter the displayed information to show only that which is associated with the revisions within the specified range. Revision numbers, dates, and the 'HEAD' keyword are valid range values.
SOURCE and TARGET are the source and target branch URLs, respectively. (If a WC path is given, the corresponding base URL is used.) The default TARGET is the current working directory ('.'). REV specifies the revision to be considered the tip of the branch; the default for SOURCE is HEAD, and the default for TARGET is HEAD for a URL or BASE for a WC path.
The depth can be 'empty' or 'infinity'; the default is 'empty'.
With some simple scripting you should be able to run this on all branches below a specific directory, etc.