I've found that all my Podcasts via iTunes (about 400 by now, I'm an avid NPR fan) are automatically marked in Genre as "Podcast". So excluding those are easy.
I'm not sure about Podcasts obtained elsewhere--but as limike pointed out, you can label anything in iTunes (use Genre or kind or composer or comments) and then set a rule to include or exclude only those.
Another trick is to include the rule "bitrate greater than xx". Or less than. Or in between.