I have a large number of random folders of photos on my hard drive, some of which have been imported into iPhoto. How do I figure out which photos have been imported into iPhoto, so I can delete the non-iPhoto copies?
If you batch drag them, as jmlumpkin suggested, a number of them will be spotted with the 'do you want to import again', and you can apply no to all, then delete the bunch. If hwoever, it accepts them, there are a few bits of duplicate finder software - none of them free (at least not that I've found), but at around $8 - http://www.brattoo.com/propaganda/.
I'd love to find something free that did the same!
Other helpful things: It may take some time, but letting faces do its thing, you will be able to group up and spot some duplicates.
I would drag each photo to iPhoto individually. It should then tell you 'these already exist in your library, do you want to import again, etc'. I would then visually check to make sure that they are in there, and then delete the folder.
If it doesn't give you that message, then you would go ahead and import them, and then can remove the original folder.