I just say do your best. If it is already in the DT some anal person will eventually point that out. Hey, I've typed in stuff that was already there, too. Even typing in an odd word doesn't always work. Say you typed in "thruppence". I've seen versions with "three pence" and "three pennies", so you might still have missed. I got it with "Revolutionary Tea" but only because I had recently listened to the recording and had a vague recollection of seeing the song in a book called "Songs that Changed The World". The only other solution would be to download your own copy of the DT and then get one of those programs that will take a large block of text and search for the nearest matches. Researchers have used them to hunt for plagiarism.