Listen closely to the opening and closing statements. Lectures often introduce the main idea within the first few sentences of the speech and underscore that same idea in a concise manner during the last few sentences of the speech.
Look for points that a lecturer writes down on a visual medium or shows through some previously prepared visual display. Many speakers use visual aids to reinforce the main idea.
Listen for numbering and repetition. Many lecturers number several important points. These points usually explain the main idea of the lecture. Speakers also usually repeat the main idea, along with its major supporting points, throughout a lecture.
Watch for the use of emphasis. If a speaker raises his or her voice, gestures more, displays dramatic facial expressions or seems more excited about a certain element of the speech, chances are this part of the speech is a key to the main idea. Another use of emphasis is transitioning from one sentence to another in a dramatic way, such as with extended silence in between or added volume attached to the new sentence. This type of transition often signals some aspect of the main idea.
Take notes throughout a lecture. Jot down possibilities for the main idea. Review your notes later to gain a clearer understanding of the central point of the speech.