Say we've produced a widget for other sites to use, like so:

<iframe src="http://example.com/whatever.php" frameBorder="0" width="200px" height="300px"  scrolling="no"></iframe>

The client would like to be able to see within GA who has embedded the thing. Is there some referer information automatically passed that I can look for, or do I need to add something?

whatever.php is already loading the analytics Javascript(we're also tracking clicks on an outbound link).

[EDIT]
Looking around a bit more, I found what seems to be a similar question on SO with an answer saying this can be found, automatically, but I still can't seem to find the information. The question's also old enough the respondent is probably referring to the old interface, though. Maybe someone could explain getting to it in the new look. (I won't likely be able to train this client to switch, deal with the old look, etc.)