How do I tell if transferring purchases from Ipad to PC worked?

I thought I did it succesfully, but when I try to update to I.O.S. 6 a popup says that I haven't and asks if I still want to continue. I tried transferring them again but the "Sync" screen on my Ipad quickly flashes & disappears and nothing else happens making me think that there is nothing more to transfer. Where can I see the transferred files on iTunes to confirm that they transferred and/or why am I getting the message that they are not transferred before I try to update the I.O.S?

 

Thanks for any help!

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