There's also Curt Clifton's
Verify Next Actions Exist script.
Adding a bit to what SpiralOcean said, if you are in context view, and you haven't disabled the feature by unchecking the "In context mode [] Show projects and action groups" option in the OmniFocus preferences, parent actions will appear as available actions, in a bold typeface (*), once all of the child actions are marked complete. That's another clue that the time is ripe to evaluate whether to add additional actions or mark the parent complete. If the parent doesn't have a context set, it appears in the No Context bin in the sidebar, but if it does have a context set, it will appear in that context, of course. Whether this appearance will be visible depends on how you've got the view set up.
That brings us around to the most important point, which is doing reviews. Using OmniFocus' review feature gives you a systematic way to look at all of your projects on a regular basis so that you can ensure they all have suitable next actions, are all still relevant, and are getting the amount of attention they merit. I'm not going to write up how to use the review feature again, but there are plenty of posts on the forum describing some of the ways you can use it.
(*) With the default style settings in the OmniFocus preferences