Compute distance between the centers of the two spheres. If it's less than the sum of the radii, then there exist points that lie within both spheres -> the spheres overlap.
def SphereCollidesWithSphere(s1, s2):
d = s1.center.distanceSquared(s2.center)
rsum = s1.radius + s2.radius
return (d <= rsum*rsum)