How does Emacs know the background color?

[Michael Herman]

I am using a special mode for handling e-mail called post.el.  In
post.el there are different faces depending on the background being
'light' or 'dark'.  When I launch emacs -nw from within an xterm with
a white background, emacs uses the face for a dark background.

How does emacs determine what the background is?

When I use -rv for the xterm, emacs still thinks the background is
dark.

On a sort of related note, is there a way to tell how emacs was
launched from the command line?  I'd like to modify my .emacs to
change settings depending on whether I used emacs -nw or not.

Thanks.

-- 
Michael Herman