Here's a bin showing listening to the iron-select event:
So we're clear, there's no list of on- event attributes. Polymer supports this as a general pattern. on-<event-name> adds an event listener for <event-name>. That event name can be a standard DOM event like
click, a Polymer gesture event like
tap, or an element-specific event like iron-select.
As Karl said, any property with notify: true also generates an event. You could also listen for _those_ with on-<property-name-changed>, like, on-value-changed, on-selected-item-label-changed, and so on.
The paper-dropdown-menu doc is a little unclear, but the meaning of the 2nd paragraph is that you can specify any menu element you like (paper-menu, paper-listbox, or an element of your own). If you create a new element here, it needs to fire an iron-select event when a new item is selected, and the event's detail.item should be the selected item.
I'll submit a PR to try and make this part of the API doc a little clearer.
Hope that helps.