Search to find blank custom column

I have two custom date columns -- #Started and #Finished. I'm trying to figure out how to do a search that returns all those books that are started, but haven't been finished. The Started is easy -- #Started:>1/1/1900. Which is, I admit, a kludge because I couldn't figure out how to deal with blank, not-blank. Unfortunately, that kind of kludge won't work for the Finished.

I looked through the Search help files, but nothing really seemed to fit. Which is making me feel like a bit of a doofus, but not as much so as I'll likely feel when someone points out the obvious that I've missed!

Thanks,