How does a doctor determine "normal-tension glaucoma"?

Glaucoma. Normal tension glaucoma is when glaucoma (damage to the optic nerve due to intra-ocular pressure) occurs and the pressures are normal. Some peoples optic nerves can not withstand "normal pressures". It will be detected by routine exam and clinical findings consistent with glaucoma, but with normal pressures. They will need to do full work up to "determine".
Visual field loss. Glaucoma is a problem in which the internal eye pressure presses upon vessels on the back of the eye gradually causing loss of vision. This occurs mostly when the pressure is elevated above defined normals. In "normal tension" glaucoma, the vessels are sufficiently sensitive that pressure defined as normal can still over press on these vessels and cause vision loss.