Look for the first sentence in the paragraph. Topic sentences, also known as main sentences, are often in the beginning of the paragraph, although this is not always the case. (See Reference 2.) Sometimes, another sentence is placed before the topic sentence at the beginning of the paragraph to provide a link to the last paragraph or to give background information. (See Reference 3.) These sentences are used to provide transition from the previous paragraph and to lead the reader into the specific idea of the new paragraph.
Look for a sentence that is more general than the others in the paragraph. Some words to look for include "several," "many" or "numerous." A topic sentence looks at the larger picture. (See Reference 2.)
Turn the sentence you think may be the topic sentence into a question. If the other sentences in the paragraph can provide information about the question, the first sentence is the topic sentence. Supporting sentences in a paragraph provide details about the subject of the topic sentence.
See if the sentence has words that are very specific. Sentences that support the main idea of the paragraph discuss details using words such as "first," "next," "third," "moreover," "however" and "for example." If the sentence contains these types of words, it probably is not the main sentence. (See Reference 2.)