You can know that they are verbs because all you need to know is that verbs are action words. The words: talk, smile, teach, laugh, study, eat, punch, fight.
They are verbs. Other verbs include linking verbs such as: is, are, was, were, will be, shall be, and etc. You will determine that it is a verb because first, it is not the one that it was talked about in the sentence. Second, it is a word that is used to refer to the action done by an object or not.