What is the right way to begin validation for data sent by a multipart/form form. I'll be receiving a file and 1 text identifier from a hidden field.

I'm not asking how to check for file type and size and all else. I can do that :-) What I'm asking is: in a normal form I usually do this:

Code:
<?php

If ($_POST)
{

if (isset($_POST['myField]) && $_POST['myField'] == 1)
{
//Proceed with the rest
}else{
echo 'Error';
}
}
else
{
echo 'Some Error';
}

?>
What's the best way to do it when it come to multipart/form data? How do u pros / experienced guys do it?

Thanks