Re: Recognize mbox files?

[Richard M Stallman]

    This mbox detection problem sounds like what we are trying to
    solve with bug#2222 (Labels). Launching rmail would automatically
    add a message on top of the mbox file, adding special rmail
    headers and thus making this mbox file detection really easy (if
    grepping for rmail headers on the first message).

It is already easy to detect an Rmail file.
Just look for the X-RMAIL-ATTRIBUTES header in the first message.