Some lecturers provide clues as to what is most important. The main ideas then may be supported or similar main ideas may follow. In addition to different types of clue words, you may find the instructor's presentation pattern is helpful.
Lecturers have different patterns for presenting material.
Listen carefully the first few minutes of a lecture and try to determine the speaker's pattern of delivery. If you know the pattern, this will be helpful in determining what the most important ideas are.