You can always tell when cats are happy. Sometimes they purr, rub against you, smile, or show their happiness through body language. Here are some happy cats!
Rubbing and purring:

This friendly calico in Beaufort, SC, just came over and rubbed herself against Dale’s hand and purred. Obviously happy for the attention!
Relaxed and asleep in a warm nest:

A sleeping cat, nestled against human legs in a warm bed, is clearly happy! (One of my Manx cats, Joia, 1985)

I think Hazel is smiling in her sleep here!
Rubbing and purring with love for catnip:

Embracing a scratching post and rubbing against catnip makes Hazel happy (not to mention “high!”)
Purring and eyes closing in contentment and trust:

Moxie, formerly my niece’s cat, is happy with the attention given to her by her human caretaker, John.
Such happy cat photos. I have two cats and totally understand. 😀