Because the cornea is the most outermost transparent layer of the eye, at the same time, it is extremely sensitive.

The cornea is therefore directly exposed to the environment; anything affecting the cornea may affect its transparency, leading to potential permanent visual impairment, and may cause severe pain.

Following methods are available:

Slit lamp biomicroscopy
Ultrasonic biomicroscopy
Endothelial Microscopy
Optical coherence tomography (OCT)
Ultrasonic pachymetry
Computer topography
Aberrometry

A wide range of treatments, depending on the specific pathology are available:

Pharmacological treatment with eye drops and ointments
Treatment of dry eye with artificial tears, oral medication, autologous serum, lacrimal implants.
Surgical treatments:
Pterygium surgery with antimetabilites and / or autologous conjunctival grafting.
Treatment of keratoconus with special contact lenses collagen crosslinking and / or Intracorneal rings
Treatment of corneal scar tissue with Excimer Laser
Full / Partial thickness corneal transplantation.
More information at:

https://www.nei.nih.gov/health/cornea