Find revision in trunk that a branch was created from

From the command line, the –stop-on-copy flag can be used to help show you where you copied a branch from:

svn log --stop-on-copy --verbose --limit 1 -r0:HEAD ^/branches/feature

(where feature is the name of your branch)

The last line of will say something like this:

Changed paths:
   A /branches/feature (from /trunk:1234)