Products for which you are supposedly the target user: tell him why you think you would not not currently use it. Products for which you are not supposedly the

Products for which you are supposedly the target user: tell him why you think you would not not currently use it. Products for which you are not supposedly the target user: you don't know enough to tell him it wont work. (edit) In all cases: you can help by pointing him to tried and test methodologies for validating ideas. e.g. Lean Startup (Reis).