If you ever forget, add --dry-run. Subversion will tell you what will be merged without executing the merge.

$ svn merge -r1752:1765 [YOUR BRANCH] --dry-run
--- Merging r1753 through r1765 into '.':

In my case I'm reminded that the first revision doesn't start on 1752, it starts on 1753! If that isn't what I want, I can change it and when the results look right, I will remove --dry-run and run it again for the actual merge.