Knol has recently added a search toolkit that lets you build advanced queries, a feature that wasn't available at launch. Many of the options from Google web search can be used in Knol: phrase search, negative terms, OR search. You can also restrict the search to titles, summaries, authors, reviews, recent knols. Google offers three options for ordering the results: by relevance, by creation date, by last modified date and an interesting "reverse sort".
The advanced search option should be a good way to discover knols related to a topic, but it fails to properly rank the knols. In a search for [Google Reader], the top result is a page copied from Google Reader's help center that has a higher rating than the second result, which is more relevant. If you don't use the advanced search, Knol shows less but better results.
A search for [the] returns 29315 results (507 edited in the past 24 hours), which should be close to the total number of knols, but many of them include content that already exists on the web, aren't very detailed or well-formatted. You can still find some good knols, but the signal-to-noise ration seems to be low. The knols that receive some attention are those that are featured on the homepage, but Google only features 5 hand-picked knols daily.