How do you distinguish between transitive and intransitive verbs?

The verb is transitive when responding to the question WHO, WHAT:

Luca legge(che cosa?) un libro Luke reads (what?) a book

intransitive verbs don't answer to the question "who" or "what", but often express a way of being of the subject and have as auxiliary verb "to be"

Io sono uscito presto da scuola I came out of school early