[gentoo-user] Portage: Show list of required files without downloading anything?

[Statux]

My mother has a computer which still accesses the internet via a dialup
connection. When I do upgrades on her system, I typically grab a portage
snapshot from my system, drive up to her house 12 miles away, install
it, check to see which packages need upgrading, then somehow figure out
(never exactly) which files she actually needs so I can get them, again,
from my system, drive those files back up to her house and compile stuff
hoping I didn't miss anything too sizable.

Question: Is there a way that I can get Portage to run through the
packages/ebuilds and, instead of downloading anything from the net, just
have it show me which files were not in /usr/portage/distfiles which
will be needed?

So if I had 8 packages which needed upgrading, which would result in (an
estimated) 6,382K of downloads, is there some way for me to have it go
through each one of those all at once, similar to --fetchonly, and have
it spit out a list of everything which it did not find on the local
system?

It seems to be simple enough and a useful feature on some level but I
haven't found the answer.

Thoughts?

-Statux