How Do I Know That I Have Lung Cancer ?

I am 42, male, smoking for17 years now. Although I'v reduced it to 5 cigs. I have constant pain in upper half of my chest , upper back between the shoulder blades that extends to my head . The pain is reduced after blurping or when the gass passes, which is very rare. The sputum is very little but very sticky,green, smell tastes very bad. I have to cough very hard to raise it, which is very painful in my back head. And I feel weakness all through my torso . I think I'v already developed cancer . What should I do to findout?? Need Help.