Recently I've shared an indepth look into how Google Now Topics Gives You Insight into Hummingbird Update by @ajkohn. Yesterday's Google Trends update also gives some insight into how Google is clustering queries and identifying + defining (named) entities.
It looks like a solid way to look into Google's knowledge base.
Another great read in this direction is Bill's Hummingbird and Author Rank Authority
The Google Hummingbird rewrite of Google’s search engine wasn’t just aimed at providing a way to better understand long and complex queries, like the type that someone might speak into their phone. It was also likely aimed at better understanding the concepts and topics written about and discussed on Web pages, and in social signals such as posts at Google+ and comments on those posts, in Tweets, in Status Updates, and other short text based messages where there might not be a lot of additional context to go with those messages...
A knowledge base that includes entities and their attributes, concepts and keywords can be useful in expanding queries that someone searches for, to show a wider range of relevant search results, like in the Probase example above.