Hacker News Reader

Another common option is a "toast notification" (see android) or other separate confirmation message to acknowledge your action.

"Sent X" or "Finished Y!" is sufficient to distinguish a failed ajax call from a successful button press

Yes but be very careful with those. You don't want to spam the user.

I remember some years ago when Ubuntu file sync would pop a notification every time a file was synced. That's a good example of what not to do.