Find Favorites on GPS that are not in Basecamp

I use Basecamp to store all my personal map/travel data. Specifically my Favorites (i.e. favorite waypoints). I keep them in lists, export the lists by name, move them to the SD card on the (Zumo) GPS, then import them. They show up in groups.  This is all good.

Occasionally when I am on the road (or not convenient for me to access BaseCamp), I will add a new favorite through the GPS interface. For example, a good restaurant, or campground, or person/business address.  Any number of things.  I don't do a good job remembering when and what I have done, so those favorites don't get entered into Basecamp.  And even if I knew how to find them on the GPS, I don't know how to get them over to Basecamp.

I'm wondering if it is somehow possible to compare Favorites in Basecamp and on the GPS to find those in one place and not in the other? I'm not looking for some sync function. But a 'differences' would be really nice. Or anything close to such a thing.  I realize there is unlikely any program to do this. But does anyone have (or suggest) even some kind of manual procedure for doing so?

I see that if I attach my GPS and click on Internal Storage, I will get a display of (all?) waypoints on the GPS.  But what can I do with that that would allow me somehow to filter out the ones not in Basecamp?

Thanks for any help/suggestions you can provide.