You would need to ask yourself if the participle is describing another noun (i.e. participial adjective) or describing an action in progress (i.e. present continuous tense, past continuous tense).
He was surprised at the sound of his own voice. ("surprised" describes "He". You have a participial adjective here.)
He is surprising her with a gift. ("is surprising" is something he is doing. Here, you have a verb, NOT a participial adjective, as there is clearly a subject and an object.)
I fell back astounded in my chair. ("astounded" describes "I". You have a participial adjective here.)
1. This is an interesting story. (“interesting” describes “story”, and is a participial adjective.)
2. Politics has never really interested me. (there is a subject and object. “interested” is a verb.)
3. The lecturer is interesting ("interesting" describes "lecturer" -- participial adjective), and I am interested ("interested" describes "I" -- another participial adjective) in his lectures.
I hope this helps.