<p>I am writing an application where I will be accessing the database from django and from a stand alone application. Both need to do session verification and the session should be the same for both of them. Django has a built in authentication/session verification, which is what I am using, now I need to figure out how to reuse the same session for my stand alone application.</p>

<p>My question is how can I look up a session_key for a particular user?</p>

<p>From what it looks there is nothing that ties together auth_user and django_session</p>

This is somewhat tricky to do, because not every session is necessarily associated with an authenticated user; Django's session framework supports anonymous sessions as well, and anyone who visits your site will have a session, regardless of whether they're logged in.
This is made trickier still by the fact that the session object itself is serialized -- since Django has no way of knowing which data exactly you want to store, it simply serializes the dictionary of session data into a string (using Python's standard "pickle" module) and stuffs that into your database.
If you have the session key (which will be sent by the user's browser as the cookie value "sessionid"), the easiest way to get at the data is simply to query the Session table for the session with that key, which returns a Session object. You can then call that object's "get_decoded()" method to get the dictionary of session data. If you're not using Django, you can look at the source code (django/contrib/sessions/models.py) to see how the session data is deserialized.
If you have the user id, however, you'll need to loop through all of the Session objects, deserializing each one and looking for one which has a key named "_auth_user_id", and for which the value of that key is the user id.