a close look at their eyes. the eyes should only be partially opened. aside from the twitching, your dog should look peaceful. pay close attention to their breathing. if it seems normal, then there's no reason to worry.
as a last ditch effort to know for sure that they're only dreaming, you can wake your pup up. the behavior they exhibit after waking up can be a good indicator of what was really going on.
say their name gently to take them of their sleep. you should never shake your dog or wake them up suddenly, especially if they were dreaming. your precious pooch may think they're still dreaming for a split 