It’s the downturned eyebrows and the slightly wider mouth. It’s the “I’m showing my teeth because that’s what a grin is supposed to look like, but I’m not smiling with my cheeks” look. Because smiling with your cheeks is involuntary, and causes the outside edge of the mouth to pull up slightly more into a “sharper” smile.

Source: Am autistic enough to have studied smile posture to be able to identify when people are being genuine.