Well, download and install
Analyzer.
Open it, click File -> New empty file.
<make sure you are not downloading or uploading anything at this moment, otherwise it will capture too many packets.>
Now click File -> New capture -> OK.
It is now capturing whatever packets comes in and leaves.
Now, open the video of which you want its url. After the video has started playing for a sec, stop capturing packets.
Now click File -> Save as ... -> save as a (Text file) .txt file.
In that text file you can do a simple search for the url. CTRL + F, and search for the url.
There are also plugins for firefox that allow you to download from many of these sites btw. It is called
Videodownloader, you can get it on the extentions page of firefox. Then you always have an icon on the right side of your browser, if you click on it, it pops up a page with links to the swf/flv/avi/wahetevr videos on that site. Simple right click save and you have the video.