Usually if the contents of a content control are deleted the placeholder text is shown. If there is no placeholder text, the placeholder text is still 'shown' if the control is empty, so you can check whether the control is showing placeholder text e.g.

Function OCC_HasText(oCC As ContentControl) As Boolean
    If Selection.InRange(oCC.Range) Then Selection.HomeKey wdStory
    If oCC.ShowingPlaceholderText = False Then
        OCC_HasText = True
    End If
lbl_Exit:
    Exit Function
End Function
Frankly it is simpler to have placeholder text associated with the control, which is the default. You may find https://www.gmayor.com/insert_content_control_addin.htm useful.