They color coded the send button! LOL. Blue and green (the two colors most people with color blindness issues can't see or distinguish between btw). In your example, that does pose an interesting question if you have activated it say on your iPod touch. I could see this being a pain. If you had to be online for it to work, it kind of is stupid. If you have to manually tell the app to revert to your phone, kind of stupid too. I think having it all as 1 unified app would be where confusion can happen. How many people would pay attention to the send button being blue or green? Perhaps someone with the beta will post about this soon.
Anyone using the beta have the answer? I would hope it would be more obvious like Facetime is.
I think there is an accessibility option that remedies the colorblindness issue