How do you diagnose a corneal perforation?

A perforation means that the cornea has been penetrated all the way through. You can diagnose this by using the Seidel test to see if any aqueous fluid is leaking out. You perform this by wiping a strip of fluorescein over the wound, and viewing how the dye appears. With a positive Seidel, you can see the clear aqueous running through the yellow stain.

To get a better idea how this looks, you might want to watch this excellent seidel video at RootAtlas.