An easy way to find all the non-MP3 songs would be to create a smart playlist of everything where Kind contains AAC (see attached screenshot), then use that to convert everything.
One thing to note, though...moving from AAC -> MP3 isn't necessarily a good idea for a couple of reasons:
- Transcoding from one lossy format to another is going to sound pretty crappy, and
- In general, lower bitrate AAC files (assuming they were ripped directly from the source) sound as good as higher bitrate MP3s.
...so, while you _might_ save some HDD space, you're more likely to end up with crappy-sounding tunes...and no one should have to suffer that.