flock-talk:

It can be hard to tell sometimes, especially if you haven’t seen that species at it’s happiest before.

Using a personal example:

Way back when Mia was clipped she didn’t want to interact with people very much, she wasn’t aggressive she just didn’t enjoy it.  She wasn’t overly interested in toys, didn’t really move a whole lot unless necessary.  She didn’t have a whole lot of energy, sat around a lot and didn’t show interest in much of anything.  

I just thought that was the way she was, maybe she was just shy or didn’t like human interaction.  Not everyone’s extroverted with their behaviours so i thought maybe Mia was just the same way.  Mia liked sitting in front of the window and just chilling all day, I figured that was just her preference.

Then her primaries grew in, got her a new cage, started up training

She was flying all over the place, engaging with toys, wanting to be with me, started showing interest in what I as doing, she was flying herself over to me for snuggles, gradually she started letting me scritch her, she started foraging and looking for things, she became more energetic.  She got louder, started vocalizing more just for the heck of it, she got brighter and she just felt happier.

I didn’t know at the time that she was so miserable until I saw the changes that were happening.  I know better now and understand what she’s like when she’s unhappy, I’ll remember that forever now and do my best to make sure she never feels unhappy again.

Now she still likes sitting in front of her window, but she plays with toys while doing it and just generally looks happier.

Different species display happiness in different ways.  Conures and cockatoos may dance around, hop, and show excitement with their people showing that they’re happy.  

The few common things are the bird’s energy, involvement, curiosity, and volume.

A happy, healthy bird, will be energetic, climbing around, flying, walking and just wanting to keep moving (of course they’ll take naps but they shouldn’t be a lump all day).  A happy bird will be interactive, playing with toys, people, always wanting to have their beak touching something or doing something with another thing or person.  Curiosity is a really major one, a happy bird will be interested in their environment, an unhappy one won’t show much motivation to do anything but when they’re happy they want to find new things they can get their beaks on, they want to find new things, they want to forage, they want to be active mentally in some way.   Typically a happy bird will be fairly vocal but this one does vary quite a bit, not all birds like a lot of vocalization, unlike budgies who typically chatter all the time, some species prefer the quiet.

Look up videos of your species and see how those birds are acting, your bird should be fairly similar in terms of energy.  Birds are very active, if your bird isn’t active then something is probably wrong.