Quick post, hopefully useful reference material ….

getSaveMode(), returns a value indicating how the save event was initiated.

execObj.getEventArgs().getSaveMode()

Note: Don’t forget that you will need to tick the option to return the context as the first parameter on your on-save event.

The following code could be used to prevent save from auto save events!

function preventAutoSave(econtext) {
    var eventArgs = econtext.getEventArgs();
    if (eventArgs.getSaveMode() == 70) {
        eventArgs.preventDefault();
    }
}