If the existance of graviton has not been proven ,how they they determine that it spins?

Answer 1

Even though the graviton is a hypothetical elementary particle that mediates the force of gravity in the framework of quantum field theory, the mathematics requires that the graviton must be massless (because the gravitational force has unlimited range) and must have a spin of 2 (because gravity is a second-rank tensor field - this is a General Relativity requirement).