"Saber Face" is a trait they share, and it's an official term that comes with meta humor. Main thing is that they have different body types, Nero being shorter and thicker. Plus their personalities are different, along with their facial expressions & body language. Their official art is usually drawn by two different artists with different styles, Nero by Wada Arco, Artoria by Takeuchi. So once you get more familiar with Fate they're pretty easy to tell apart.