How can I tell which Events I've already reviewed?

To help you keep track of which Events you've already reviewed in a search result, a "ribbon" is added to the upper left corner of the Event when the Event has been reviewed.

If you "scrub" an Event by moving your mouse back-and-forth over the Event, changing directions 3 or more times, then a semi-transparent "reviewed" ribbon appears.

If you actually play the Event, then a fully opaque "reviewed" ribbon appears.

For example, the Event on the left has been played, whereas the Event on the right has only been scrubbed: