How to separate single photos from that have been put into an album

I always import and categorize my photos into albums immediately. But I've accidentally imported photos without putting them into an album so they don't appear in the 'events' section. They do however appear in the 'all photos' section along with thousands of other photos. Is there a way to separate photos that are already in an album from those that are not yet in one so I can easily see which ones I haven't organized and sort them from there?