Here is the API you are looking for: -- https://archive.org/wayback/available?url=example.com×t... -- this will show the oldest timestamp of a archive that the wayback has. The trick is to set the timestamp to be /really/ old and it will show the first snapshot it has.
I don't think you can tell if "the text" of the page has changed since then without manually looking though. I'm not sure how you'd solve this technically, it's probably more than just telling you if the html bytes are identical, since small changes probably happen all the time to "the same article".
Or it might be good enough, because this kind of content is rarely updated at the same url?
I wish Wayback had an indicator for how much the page has changed from one capture to the next. A simple count of the diffs should give you a good idea.
Or it might be good enough, because this kind of content is rarely updated at the same url?