Parrots are very expressive, and if you live with one for months/years, you'll learn its moods. Not hard to see how a parrot responds to something new.

I think it's very hard to objectively measure and give it a concrete number, but anyone who have kept a pet (dog, cat, parrot, pig or otherwise) can usually tell if their companion is happy or not, as they have bunch of signals they give us throughout their lives. With dogs, you can usually tell by the ears if they're curious or defensive, while the tail tells you if they're happy.

I have a parrot. She cannot talk. But she speaks and socializes on speakerphone and facetime. She definitely recognizes some callers and interacts with them during the call. It makes her happy - though at some point she'll sort of run out of attention and start doing her normal bird stuff.

From the look of his face, that parrot looks very happy.