Popcorning is a big one. However, as my pigs have gotten older, they don't popcorn as much but I know they are still happy. Squealing can be a happy sign but so are things like running to the cage when you come home. They generally act cheerful or giddy as well. Happy pigs enjoy eating and enjoy exploring. If they are bored, they sit around a lot and simply look bored. Make sure you are stimulating them by having a veggie "scavenger" hunt, changing the layout of their cage, give them new veggies to try, and supply them with different toys like boxes with pieces cut out and different sizes of bags. When my boys are happy, they have more of a "hop" when they walk as well. A pig who is healthy, well fed, paid attention to and loved is a happy pig!