Transitive verbs always have a direct object. You can determine if a verb is transitive or not by searching for the direct object in the sentence. Do this by rephrasing the sentence to ask, "Who or what does the (subject) (verb)?" For example, reformulate "The boys played football" to "Who or what did the boys play?" Because there is a discernible answer, "football," the verb is transitive. In "The teacher thought for a while," the question "Who or what did the teacher think?" has no discernible answer. The verb is therefore intransitive.
An indirect object can look like a direct object, and might lead you to identify an intransitive verb as transitive. The indirect object describes where, when or in what manner an action is performed. Because it can come right after the verb in a sentence, it can look like a direct object. In "Patrick walked to the store," "store" is the indirect object. Because the question "who?" or "what?" cannot be answered, "walked" is used intransitively in this sentence.
Only transitive verbs have active or passive voice. A sentence using voice must have both a subject and a direct object, and therefore has a transitive verb. "Martha made a sandwich" is transitive because it contains a subject that performs an action on an object. Identify this quality by rephrasing the sentence as passive, beginning with the object: "The sandwich was made by Martha." You can't do this in the sentence, "Paul listened carefully," and therefore the verb is intransitive.
Linking verbs that connect a subject to its complement, something that describes the subject, are always intransitive. Some common examples are "is," "was" and "become." In the sentence "The light is green," "is" is intransitive, because it only links a property "green" to the subject ("light") and has no direct object. Look for verbs that only serve to describe the subject, rather than illustrate an action performed on or to an object.