static inline GLfloat Vertex3DCalculateDistanceBetweenVertices (Vertex3D first, Vertex3D second)
{
    GLfloat deltaX = second.x - first.x;
    GLfloat deltaY = second.y - first.y;
    GLfloat deltaZ = second.z - first.z;
    return sqrtf(deltaX*deltaX + deltaY*deltaY + deltaZ*deltaZ );
};

Works out the distance between two objects in 3D space, you'll need the Vertex3D struct also in my snipppets

OpenGL Distance to object

Other