Well you and I, after having garters for YEARS can catch those subtle differences in mood and such. Also, those differences vary with individual snakes. My female was very expressive. My male was not. Someone who's never kept snakes once asked me "how can you tell?" referring to the snake's mood, and their fixed expressions. "Looks like he's smiling to me" when it was clear to me, that a particular snake was not happy at all.

Not everyone can see that. Not everyone sees the differences, and you have to admit, they are very subtle. Even for you and I, it's hard to tell with a new snake, or a species we have never kept. You have to get to know them. If you've never seen a particular snake happy or in a good mood, or distressed and in pain, you don't really know what it looks like. An angry or annoyed snake is usually pretty obvious. Not so with the other moods, or a snake in pain. I guess that's the point I was trying to make.

Hey, you know, it just occurred to me. It could be something as simple as a little pine tar, or sap. I saw that once on a garter's lips.