It is by listening and evaluating the speaker's questions and responses. When the speaker asks a question, he is surely making a point to the audience through his responses. By that kind of communication, the speaker should have given his purpose to the audiences.
1. An article in a news magazine titled " Fences wont stop illegal immigration"
Purpose: to persuade
2. "The iPhone versus the Android ; Which one is best ?'' - an article in a computer magazine
Purpose: to evaluate
3. An article in a newspaper titled " Entire city Installs solar panels on its Homes"
purpose: to inform
4. "Current trends : The Decline of Marriage '' - an article in a popular science magazine
purpose: to analize
