Button In Smart App?

I have a dual button. In a smart app I can detect whether a button was pushed or held. But I can’t seem to figure out how to tell which button was pushed.
I tried device.button, device.numberOfButtons, device.value[0], etc.

Someone give me hint?

subscribe(buttonDevice, “button”, buttonEvent)

def buttonEvent(evt) {
def buttonNumber = evt.jsonData.buttonNumber
}

Thank you. That did the trick.

Is there somewhere that would be documented?