logged in or NOT

Hi,
I am developing web application to show up photos from flickr and
upload photos to flickr. While uploading a photo I am facing a small
glitch.
I use photo upload api thats works fine. For uploading we open up a
small overlay , user browes the photo he wants to upload and clicks on
"UPLOAD" button. If he is already logged in this goes fine. Now as per
authentication protocol if he is not logged in he is requiured to do so
flickr signing page opens up in that small overlay. 
What I want -->  if the user is not logged in he is redirected to the
login page in the BROWSER itself.
For that I should know whether
http://flickr.com/services/auth/?api_key=[api_key]&perms=[perms]&api_sig=[api_sig]
is redirecting me to my callback script with frob OR /signin/choose
If I know this I can open up the overlay or open the redirection page
in browser accordingly. In one sentence I need to to whether caller is
logged in or not (flickr.test.login like function but with NO permission)
I hope I am able to explain what I am looking for.
Regards,
Amlan




 
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