So you want to hide an element on the page when a file is selected and show it when it is not? Or do you want to rerun validation every time a file is selected? Either way, the basic principle is the same. Wire up the OnClientFileAdded and OnClientFileRemoved events to a single event handler, and then write some code like the following:

function OnSlickUploadFileChanged(file)
{
if (kw.get("<%=SlickUpload1.ClientID %>").get_Files().length > 0)
// at least one file is selected
else
// no files are selected
}

Replace "SlickUpload1" with the ID of your SlickUpload control and the comments with the logic you want to do (hide element, rerun validation, etc.).