Is it free? If it is free and it is not named Dmoz or some other really well known directory, then it is probably crap.
Does the directory mention any SEO-related buzzwords like PR, dofollow backlinks, high authority links, etc.? If it does, it is junk you should avoid. If it has not been targeted by Google already, it will be.
Is the directory auto-approve? If it is auto-approve and not moderated, it is almost certainly junk.
That is a quick screening you can do to eliminate most directories.