What are common symptoms of glaucoma before diagnosed?

Not usually: Glaucoma is often silent as you are not able to tell when there is slight damage to the visual field. Furthermore, when eye pressure is an issue, high pressure in the eye, up to about 30, can feel totally normal. No pain, no visual symptoms, or external signs may be present. Only in the late stages does the field constrict to become symptomatic. Prevention is always better so best to get exam.

Usually none: "The (silent) thief of sight" is aptly applied to the most common form of glaucoma called open angle. There is no early warning sign or pain to give you concern. A family history of the disease, as well as high intraocular pressure, African or Hispanic ethnicity, steroid use, eye injury, thin cornea, high myopia, hypertension are important risk factors. A dilated exam is essential for diagnosis.