If we are able to add multiple events to buttons, the only way we can see those actions is to pro-actively switch to that event (via the Event drop down in the InDesign Buttons and Forms panel) or by exporting an interactive PDF and viewing the actions in the button's properties dialogue box in Acrobat.

Ideally, if an object is turned into an interactive button, the panel should change to reflect that. Within the new space, if a button is applied an action via the event drop down, line items should appear similar to the way actions aggregate. Another method would be to have the Event drop down menu show a small asterisk or indicator that there is an action applied to the specific event in the drop down list.

So if I add an 'On Roll Over' event action AND an 'On Roll Off' event action, both items in the Events drop down menu will either be highlighted or have an asterisk indicating that those event currently have actions applied to the selected button.