When a dog is happy they will usually be in a playful mood.
Also the dog when happy will usually wag it's tail more often showing you that they are in a happy mood and they will usually be very active.
Another good way to tell that a dog is happy to look at the dogs body language.
A dog instinctively uses its entire body to communicate. There are a plethora of ways to see that the dog is happy by looking at their body from head to toe.
A dog's posture should be relaxed and comfortable. Their eyes shouldn't be opened too large or shut tight. The mouth should be slightly open.
Some dogs when happy will appear to be smiling because they're comfortable and happy.
Even the ears can be used to determine emotions.
Most dogs pull their ears back when happy or leave them completely relaxed.
Overall, a calm and relaxed body means that the pooch is living cheerfully.
A dog may also lick you in the face or want to jump on you when the dog is happy.