In my PreK room, it looks like students putting their name card with their faces on either happy or not happy as part of morning routines. They can change it throughout the day. I make sure to check in with those not happy students with a short chat so they can express their feelings. Other ideas could be for students to use a happy/sad clothes pin, turn a smiley/sad emoji or have a white board where kids can sign in. Teachers can speak with students privately who are not feeling happy and find out why. Just having a minute conversation and showing you care can do wonders for the students' moods and build stronger student/ teacher relationships.