Generally a clinical diagnosis
Fluorescein test followed by tonometry:
Fluorescein test if concerned for abrasions, corneal damage, foreign body, globe rupture
Tonometry of both eyes if concerned for acute angle closure glaucoma, uveitis, hyphema, recent history of trauma to eye
Fluorescein test if concerned for abrasions, corneal damage, foreign body, globe rupture
Tonometry of both eyes if concerned for acute angle closure glaucoma, uveitis, hyphema, recent history of trauma to eye
Keratoconjunctivitis
Viral conjunctivitis
Conjunctivitis 
Bacterial conjunctivitis
Conjunctivitis
Allergic conjunctivitis
Allergic rhinitis
Acute angle closure glaucoma