Is there a way to tell when the trash was last emptied?

Open a new Finder window. From the FInder's View menu, select Customize Toolbar… . Drag and drop the Get Info icon to the Toolbar just left of the existing Action button. Click Done on the customization panel, and close the Finder window. This Get Info button will also be useful for file selections.

OS X does not log an Empty Trash event. You can click on the Trash icon in your Dock, and if the Trash has been recently emptied, you will be staring at a blank Finder window. Now, you can click that Get Info button on the Toolbar, and the displayed modified date will be when the Trash was emptied. If you add more items to the Trash, this modifed date will be when the last item was added.

elaine100 wrote:

Is there a way to tell when the trash was last emptied?

Ugly, but workable: Time Machine backups will provide you with a way to determine that, within the resolution of your backups.