A body is moving in a perfect circle at a constant speed. are there any forces acting on this body? how do you know?

Yes, there are forces acting on a body moving in a perfect circle at a constant speed.

Although the speed of the object is constant but the direction is changing continuously. A force is needed to change the direction and that force is called as the centripetal force. So an object moving in a circular path at constant speed is acted by centripetal force.