Looking at this post, it would appear that there are new event entities created ex event.some_device_button and those would be used to trigger the flow. You should be able to use an event state node to see changes to the event entity.
Edit: After reading this WTH about event entities it seems that this may not trigger a second time if the entity repeats the same action. i.e. button is pressed automation works, pressed again a minute later the node doesn’t trigger.
If you see this behavior, try un checking the state = state radio box in the event state node.