file uploaded check

[eluser]pendalpusher[/eluser]
I am trying to put a file input on a field, that the user can use to upload a new replacement image if they want.

How do i access the field to see if it is empty or full before trying to upload the file?

i am using

Code:

<?=form_upload('userfile');?>

i just want to see if the field is empty when it is posted.

Thanks,
P