The sky looks threatening.

Looks - Intransitive verb.

Threatening - Complement.

Here 'looks' verb here describes a state, so it cannot have an object. "Threatening" is used as a complement of the verb.

A transitive verb has an object that receives the direct action of the verb. Generally, a transitive verb takes a single object.

An intransitive verb does not require an object. It can make a meaningful sentence without taking a direct object. 

There are some intransitive verbs that require a word or a phrase to make the sentence complete and make sense. Such verbs require a complement and are called verbs of incomplete prediction such as is, become, seem, turn and so on. Here, "looks" is a verb of incomplete prediction.