We know that participles can function as adjective. It’s easy to identify them when they function attributively. Examples:
The boiling water was too hot to touch.
The growing concern over climate change is evident.
They avoided the broken glass on the floor.
He quickly cleaned the spilled coffee from the table.
However, when participles are predicative, they can be confused with participles as verb. Examples:
Her instructions were confusing. [Adjective]
Her instructions were confusing the students. [Participle]
The lecture was boring. [Adjective]
The lecture was boring the students. [Participle]
The windows were shattered by the storm. [Participle]
The windows were shattered. [Adjective]
In such situations, it can be found out whether the word is adjective or participle through one of the verb features of participles: a transitive verb takes object. In Her instructions were confusing, confusing can’t be participle because it has no direct object. So goes with The lecture was boring.
With past participle (the third pair of examples above) though, two situations arise.
1. Sentence with by-agent
If the sentence has a by-agent (agent that performs action), participle functions as verb. The sentence The windows were shattered by the storm has a by-agent (storm), implying shattered is a verb. That’s because, with the by-agent, the sentence is essentially The storm shattered the windows (active voice), which establishes shattered as verb.
2. Sentence without by-agent
Sentences in passive voice can be written without agent. Examples:
The kids were exhausted.
The audience was interested.
The windows were shattered.
In such situations, without context, you can’t tell if the participle is adjective or verb.