Huh? Can't you very easily tell by checking if there is a light strip on the side?
Ah - I see a border on some. You're right, that does distinguish it (and thanks) but my reaction there to the design is: wow, by so little. It's hardly intuitive or clear: the same representation for two completely different things where the only difference is a few left-side pixels :(
I'm not sure what's displayed if the app shows the middle portion of an event that extends for more than one week - maybe a rectangle, if the rounded corners signify the beginning/ending of an event.