How can I tell if my betta is happy?

Last night I put him in a 10-gallon tank with some silk plants, rocks and some ornaments, a heater and a water filter, he has his tank light on during the day and in the evening it gets shut off. I am feeding him betta pellets twice a day like the container says but he doesn't seem to be doing much swimming today. Yesterday when I put him in the tank he was swimming all over the place, flickering his tail and swimming under his filter.