Birds are happy when they are healthy and well-adjusted but, in reality, it's very hard to say if they are really happy or not. We think that because a bird sings, whistles or talks, this must mean the bird is happy but this is nothing but wishful thinking. We just don't know how to read them correctly... Grays are one of those species that, if they are unahappy, they start plucking themselves and, if something is not done about it, they will end up self-mutilating.
I have one that is happy and one that is unhappy and for which I will start looking for a new home soon. The happy one is fine in every way, she eats well, she flies around (she doesn't live in a cage), she doesn't pluck or scream and asks for my attention when she wants it. The unhappy one is scared of everything and, unfortunately, I can no longer help her get over this because there are too many animals in my house (she has become scared of other birds) - she doesn't pluck but she tends to scream (not loud but persistently) if there is another bird perching on her cage and shies away from my hand when I clean it.