Checking Login in

Question

Hi guys,
I am making an e-commerce website. When user want to log in he enters has username and password and that is checked against the database that is he really a registered user or not if register then make a cookie on the users pc. Now when user want to buy something when he click buy button i want to check that either he is logged in or not. Can somebody please tell me how can i check that user is logged in or not. Just tell me the logic. Or if possible provide an example. Thanks in advance.
Zeb-ur-Rehman