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The below is a little slow, but it gives a nicely formatted list of packages that pip is aware of. That is to say, not all of them were installed "by" pip, but all of them should be able to be upgraded by pip.
$ pip search . | egrep -B1 'INSTALLED|LATEST'
The reason it is slow is that it lists the contents of the entire pypi repo. I filed a ticket suggesting pip list provide similar functionality but more efficiently.
Sample output: (restricted the search to a subset instead of '.' for all.)
$ pip search selenium | egrep -B1 'INSTALLED|LATEST'
selenium - Python bindings for Selenium
INSTALLED: 2.24.0
LATEST: 2.25.0
--
robotframework-selenium2library - Web testing library for Robot Framework
INSTALLED: 1.0.1 (latest)
$