WCF/WF - How to come to a conclusion that a requirement has to be developed as a service

I have a requirement and on what basis i should conclude that the requirement has to be created as a web/wcf service instead of normal code or .dll file.

Typically a requirement is developed as a webservice when you want remote clients to be able to connect to the business logic. Usually (but not always) these clients will use the web -http -- to connect to your business logic.