When you put in the feed URL, it'll tell you if someone's already created a feed for that URL -- though you can still end up with duplicates when a blog (or whatever) has feeds in multiple formats.
If you have a good idea what title someone would have given it, you can type it in directly to see if it exists; all Dreamwidth feeds come in the format http://title-feed.dreamwidth.org/ , e.g. xkcd_feed.
You could also check the grand list of the 1000 most popular feeds (which currently goes down to feeds with only two subscribers, so it's probably still pretty comprehensive):
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If you have a good idea what title someone would have given it, you can type it in directly to see if it exists; all Dreamwidth feeds come in the format http://title-feed.dreamwidth.org/ , e.g.
You could also check the grand list of the 1000 most popular feeds (which currently goes down to feeds with only two subscribers, so it's probably still pretty comprehensive):
http://www.dreamwidth.org/feeds/list