"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.
There's also an official Arknights anthology manga, here's the Oneshot about Silence, Saria, and Ifrit taking a family vacation.
Edit: The link will give you a spoiler warning btw, that's because it's a chapter later in the anthology but I believe they're all one-shots? Though I might be wrong, and apologies if I am.