Confirm Upload

I’m finishing up a flash form that will have an upload field to it. Once I click on the upload, javascript calls a popup with a regular html form on it. What would be the best way to check, from flash, if someone actually did upload a file or not. Would I have to send javascript variables to flash once the upload form is submitted successfully or maybe call a PHP script that would check if a file exists before going on? Any other ideas?

Cheers,

ghjr