"Going to" and "Will"

How to Differentiate between "Will" and "Going to"???

• When you just made a decision

         
         e.g. " I know! I will cook instant noodles".

                                                                 

• Later,

        
e.g.  A: "Hello! What are we going to have for   dinner?

        B: "We're going to have instant noodles"