You can do this super easily if you’re using a relatively newish version of Subversion (1.5 or higher, I think) with the mergeinfo sub-command.
svn mergeinfo --show-revs eligible svn://repo/branches/your-branch-name svn://repo/trunk
This will show you all the revisions that are eligible to be merged to the trunk from the branch “your-branch-name”.
Source: http://svnbook.red-bean.com/en/1.5/svn.ref.svn.c.mergeinfo.html