There is no way for them to know what bits are sourced correctly without going and finding the source, whether it's cited a dozen times or once.
No but they will know the bits that no one is claiming are sourced.
Which isn't particularly useful information. It's just the low hanging fruit, most of the work required in verifying articles are correctly sourced is checking the sources actually exist and say what it is claimed they say. At the moment, we do very little of that kind of work. I think that's because it is so much more work than just saying "there is a footnote next to the sentence, that'll do" and just worrying about those without footnotes. The other problem is that there is currently no way of marking a source as verified. I can't think of a simple system to do that which isn't open to major abuse (to work it needs to be open to everyone, an admin-only wouldn't work, and opening it to everyone makes using sockpuppets to verify fake sources you've added all too easy).