How to know if my dog is interested or not !

One of the best ways to see your dog’s capacities and interests is simply to provide a lot of possible things to interact with.
Wiggle a string in front of your dog’s nose along the ground; stash a treat in a shoebox; or invest in the many creative dog toys that are marketed.
 A rich set of things to burrow into, nose, chew, bob, shake, pursue, or watch will engage your dog—and keep him from finding his own burrowable and chewable objects among your possessions. Outside, agility training or some simulacrum obstacle course is a well-defined way to engage and interest many energetic but driven dogs.
But interest can be spiked simply by a weaving, smell-laden path, or the unexplored reaches of a field.

Dogs like both the familiar and the new.
Happiness is novelty—new toys, new treats—in a safe, well-known place. It can be cure for boredom, too: the new requires attention and prompts activity.
Hiding food to be searched for is one example: they must move around to explore the space, using nose and paw and mouth together. You need only watch the exuberance of an agility dog on a new
course to see how good new is.