I want to know as to how to find rotational symmetry of an object and not the defination of rotational symmetry.

Rotation means turning of an object about a fixed point. The fixed point is known as the centre of rotation. The angle of turning during the rotation is called the angle of rotation. If an object looks exactly the same after rotation, it is said to have rotational symmetry. 

So, in a complete rotation, the number of times an object looks exactly the same is known as the order of rotational symmetry.

Now, the rotational symmetry of an object is calculated as:

Here, in each rotation at 90° the obtained figure looks exactly the same as the previous figure. 

So, in a full turn there are precisely four positions when the figure looks exactly the same. Therefore, it has a rotational symmetry of order four.