I'm often in the same situation, trying to track down forms that are unused. The way I found is by creating a smart folder that filters by last entry date.
Which is okay, although not as good as sorting by last entry date as you have to play with the filter date a little.
What would be even better for me would be MachForm recording the last time forms were loaded at all. A form might not have recent entries just because it's not used very often. But seeing the last time the form was loaded can tell you that a form probably isn't even embedded on a page anymore.