On previous versions of Stitcher on iOS, if you played something past the 90% mark of the show and press pause or go to something else, it was considered "heard." If you played something and tapped skip anywhere in the episode, it was also considered "heard."
In version 6.0 and later, if you play something that is under 20 minutes past the 90% mark of the show and press pause or go to something else, it is considered "heard." If you play something that is 20 minutes or longer past the 2 minutes remaining point and press pause or go to something else, it is considered "heard." If you press skip, and you are not in the last 10% or 2 minutes (based on above rules), then we simply set a bookmark at that point and don’t mark it heard.
To force mark an episode as "heard" or "unheard," you can long press on the play/pause icon (iOS only).
A full blue circle with the Play icon in it means an episode is "unheard," half-circle means partially heard, and an empty circle indicates that the episode has been heard.