Before I start, please forget my ignorance when it comes to MIME and basic mail structure.
I'm looking for a general way to tell the difference between the latest e-mail reply vs all of the extra replies which are stored in the text body.
For example ThunderBird puts giant
------------------------ Forward -----------------------
or
------------------------ Reply -----------------------
banners, but g-mail doesn't seem to. I read this RFC but it doesn't seem to cover what I want. I'm also looking for a way to remove signatures from e-mails but I'm assuming there is no standard when it comes to handling them.
As far as I know, there is no specification to that. So, the only way to do that would be to track the message ids and then diff the messages against each other, to find out, which parts were part of an earlier message.
That is what the mac mail client does.
But that does not seem to be, what you had in mind.
So: as to my knowledge: There is no magic marker in the body for what you are planning.
As for the signature:
It is good practice to have a double minus + newline before the signature. But you cannot always rely on that.