An article talking about the existence of verbal adjectives.
In earlier forms of English, such as in Jane Austen’s day, it was in fact customary to use the verb to be as an auxiliary in active present perfect constructions, especially with verbs of motion, as in Mr Bingley is come! (I think in French this is still the case).
However, I think that in usages such as Are you finished? and While I am gone… the words finished and gone are actually adjectives rather than participles, in the same way that excited and amazed are adjectives, not participles in passive constructions, in sentences like: The crowd was very excited; I am more amazed than ever. In other words, not all –ed words are verbs, although they may have started life out as verbs. And some –ed words were never verbs but, by analogy, follow the pattern: participle-as-adjective + noun, as in a broad-shouldered man, or a pair of left-handed scissors.
As to where you get answers to these questions, I ended up consulting the dictionary (the big red – Macmillan English Dictionary – one of course) and sure enough, finished and gone are adjectives as well as past participles.