When a sentence contains a form of "to be" (am, is, are, was, were, etc.) followed by a past participle, it can be either active or passive.
It is passive if there is an action upon the subject. That is, it is passive if an agent is expressed (in a "by" phrase) or implied (no "by" phrase).
If the past participle only represents a state of the subject, the sentence is active.

The story was read by the teacher. (The teacher is the agent.) - PASSIVE
A lot of books were sold today. (No agent, but somebody had to sell the books, so there is an implied agent.) - PASSIVE
I am bored. (My feeling is that of being bored. I am in the state of boredom. There is no implication that somebody actually took action to bore me, i.e., to make me become bored.) - ACTIVE

There are some sentences which are genuinely ambiguous with respect to voice.

The door was closed. ACTIVE OR PASSIVE
(State of the door or implied agent acting on the door)

We don't know if the door is in a closed position or if somebody (an "agent") closed the door. Was there simply a closed door there (and nobody there who did anything to close it)? Or did we see somebody take the action of closing the door, that is, did we see an event take place? The sentence can be used to express either idea, so we can't say if it's active or passive.