Generally, you will be able to tell when a beardie is happy. They will be moving around, perking up when you walk in, sometimes, as I’ve noticed, they will have their tail sort of in the air. They will be eating regularly and will take food whenever they are hungry. There will be no stress marks, which are oval-shaped patterns on their stomach they develop when stressed. If your beardie is stressed, it might have something to do with its living conditions, or if there is another beardie in the tank. NEVER house more than one beardie together with another. It is a big stressor for them as they feel more confined to a smaller area. If problems are serious, I would suggest posting on the Beardie ER page and visiting your local vet. During brumation, they will sleep more and not move around for a few months, but still check for stress marks and give them a bath once or twice a week.
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You have some good points and I'd like to add on a bit. If they are generally active and alert, they are usually pretty happy.
The "antenna tail" that sticks up when they run around is another good sign that they are content and exploring their environment.
The oval marks on the bellies of young dragons are nicknamed "stress marks", but I disagree with this term. They are very common with young dragons and tend to correspond more closely with them darkening to bask. Color change can certainly be an indicator of mood, but it is often shown in the beard. A dark black beard tends to mean irritation (for a variety of reasons), or could mean distress if they are also acting lethargic or sick. A brightly colored beard, often with accents of orange or yellow, tend to indicate a good mood and a happy dragon.