You can tell if a conduct is proper or not by seeing what would happen if everybody behaves like that. Jesus gave us the commandment to love our neighbor as ourselves, more specifically to love our enemies, not judge but always forgive others. If everyone follows this teaching, the world would have instant peace and utopia. To me, this "proves" that Jesus taught the truth. Since he also practiced what he preached by dying on the cross, demonstrating the greatest possible love for us, he is clearly God.