A lot of times it can be all over somebody's face and body language.
Self awareness is hard. I used to take selfies to help neutrally assess my own mood. A lot of the time I was feeling something and unaware, but I could tell from the photo.
I think that's what makes it so frustrating. Especially when you know the person well, facial expression can be a really good indicator. The main problem IMHO comes in when people try to read into facial expressions of people they don't know and draw wide conclusions from it. If people mainly saw strangers looking unhappy and thought, "oh wow that person must be having a bad day" it would probably be a net positive in society. But I have no idea how to convince people to think that way.
What were you feeling that you could only identify by your outward appearance ? You don't have to look happy and smiling all the time, in many cultures ( Germany, France ) people actively distrust people who do this.
But as I recall, one of the biggest things I would notice is if I hadn't gotten very restful sleep.