It is often the first sentence of the paragraph.
Example
In this paragraph the topic sentence is the first: Motivation is one of the key factors that determine the rate and success of L2 attainment. It provides the main incentive to initiate learning a foreign language and later the determination to persevere and sustain the long and often difficult learning process. Without sufficient motivation, even individuals with the best of abilities cannot accomplish long-term goals. Teachers working in state schools are first and foremost supposed to teach the curriculum, but we cannot ignore the fact that this cannot happen without motivating our learners. In addition, adolescent learners come with their own emotional and psychological baggage and interests making the task of motivating them one of the greatest challenges for teachers.
In the classroom
Teaching learners how to identify topic sentences is a good idea, especially for exams. Learners can be encouraged to highlight the topic sentence in each paragraph as they read. An effective extension activity is to then use these as the basis for summary writing.
Further links:
https://www.teachingenglish.org.uk/article/teaching-exam-based-writing-skills
https://www.teachingenglish.org.uk/article/planning-a-writing-lesson
https://www.teachingenglish.org.uk/article/shakespeare-comedy