You’ll know when your cat is happy if you are a committed and observant cat guardian. However, we don’t really know for sure that cats feel the full range of the emotion of happiness. For instance, cats don’t giggle or laugh 😉 If we are honest, domestic cats need the sort of scientific research that dogs have received. And only in recent years have scientists talked about cat emotions. Also cats are undemonstrative. They tend to keep their feelings to themselves. These factors are a barrier to answering the question in the title.
However, we do know that cats can be content. Contentment is a quiet form of happiness. We can see contentment on their faces, in their demeanour, their body language, their vocalisations and the slow blink. Some people think that cats smile. I don’t quite believe it. Although faint smiles may occur. But permanently smiling cats and grumpy ones have a facial anatomy which produces this effect.
The classic sign of happiness, the cat purr, is more complex than simply telling the world that he/she is content. However, under circumstances when a cat should be content, the purr confirms it.
Kittens play a lot. A sensible reason why they do so is because it is fun. As play is fun, it is reasonable to assume that when kittens are playing they are happy.
A cat’s facial appearance will change subtly when feeling a range of emotions from anxiety to contentment or just plain fed up. Body posture is also evident. Usually a cat experiencing chronic pain produces a distinctly tight, pained facial expression and clear behavioural changes. These are less obvious than in humans but present nonetheless.
The picture below shows Matt Cutts (a senior Google executive) with his clearly content cat:
The picture below shows an unhappy street cat in Beijing, China:
We can also recognise the mental states which indicate that a cat is unhappy. This helps us to isolate the times when he/she is happy. You can tell when a cat is anxious or uncertain, when he is fearful and so on. These signs come from body language and behaviour such as hiding and crouching.
How can we be specific when answering the question: How do you know when cats are happy? A cat will be content when he is clearly relaxed and in the company of his human companion with whom he gets along very nicely (as opposed to a bad cat owner). Under these circumstances he may respond to some words from the human with a slow blink. This certainly indicates contentment and a relaxed state of mind.
An observant cat caretaker will understand her cat’s behaviour and habits. She’ll know when her cat is content or not. Therefore the question in the title is asked by people who don’t understand cat behaviour and traits.
We can also make presumptions. If the home is very cat friendly and the cat is well cared for, we can presume that he/she will be content at least to a certain degree. Contentment, as stated, is a form of happiness. The opposite applies, namely, in unsuitable households the domestic cat may well be in a permanent state of anxiety. If you look into the faces of street cats in less well developed countries I believe you can sometimes see unhappiness. You can see the harshness of life and the difficulty in surviving. Contrast this with the expression on the face of a well cared for domestic cat.