Do they need to put some type of dye before they diagnose it as a scratched cornea?

Yes: Yes they use an orange dye called fluorescein.

Yes: A dye called flourescein is used to show the scratch in the cornea. After the dye is instilled, a special black light is used to see if there is any scratch. The eye glows a bright green with the dye and the special lighting. The dye is then rinsd out so as not to casue any further irritation of the eye.

Not always necessary: Dye, under topical anesthesia, is used to identify smaller lesions and measure the extent of larger lesions. The diagnosis can usually be made without the addition of the dye although that is standard procedure to document the scratch (abrasion).