Finding a form element name

I am on Drupal 5.23 using the modules Prepopulate 5.x-1.5 and Video 5.x-1.5

I am trying to pre-populate the fields for adding a Video URL node.

Pre-populating the title works fine:
?q=node/add/video/url&edit[title]=Oranges

Then I tried to unsuccessfully pre-populate the form field called "vidfile", like this:
?q=node/add/video/url&edit[vidfile]=Strawberry

Taking a guess, I changed the URL to this, it works:
?q=node/add/video/url&edit[video][vidfile]=Strawberry

Obviously I got lucky, but in the future how do I identify the naming of form field inside a parent?

Using Firebug to scan the HTML elements, I cannot find any name or ID of "video" occuring before the vidfile form field. This is telling me that Prepopulate does it's work before the HTML page is rendered, when Drupal compiles all the data. Is this true?

Anyways, now I have another field in add node Video URL... this field is called Video Thumbnail and is a file upload field. I am trying to pre-populate that as well.

When inspecting with Firebug, this file upload field has the name "files[image]", so following the advice of the module documentation I tried this url unsuccessfully:
?q=node/add/video/url&edit[files][image]=Blueberries

I also tried some variations/combinations of [video], [thumbnail], [file], etc.