What is the code to check whether user has attached something ?

I have in aspx/html this simple html file upload
<input type="file" value="myFileName2" name="myFile2" >
This generates a "Choose File" button.
When user click "Choose file" , browse his file and click OK a name appears on the page which is all fine at this moment. However, there are some case where users do not click this "Choose file" button.
How can I check in my code whether user has click that button (Choose File button not Submit button) ?
For some reasons I cannot use the separate ASP Upload control and need to use HTML file upload
Thanks