The working copy is revision BASE. The latest copy from the repository is revision HEAD. This will compare your working copy against the HEAD revision:
svn diff -r HEAD <file>
Actually that’ll spit the changes out in reverse, i.e. it tells you how to go from HEAD to BASE. So technically you want:
svn diff -r BASE:HEAD <file>
Can you spare the keyboard strokes? Only you and your deity know that answer.