I'm guessing it has something to do with how quickly the user returned back to the search results. For example, you're searching for something specific and click on the #1 result. If you visit site #1, go back to the results, click on #2 then visits to #1 could be seen as a "bounce". Of course this could theoretically be gamed, but that's for another discussion.

Oh they have a few ways actually.

- "Clickstream" data from Chrome. Sorry I don't have the source in hand, but I remember that Rand Fishkin was talking about it some time ago.

- Statistical analysis on the SERP (eg. if you search for something, click a link, go back and pick another result, that is a bounce)

- This is just speculation, but I'd say their ad network can be a source too

According to a comment in this thread: "They also know when you hit the back button to come back to the search results", so I guess they can determine roughly how long you spent on a page?

By seeing how long after you clicked on a search result you get back to the results page.