Often none. With slow pressure rises from open-angle glaucoma there are really no symptoms only late vision loss. With acute closed-angle glaucoma there is pain and pressure with a red eye. Often patients notice "steamy vision" with rings around lights from corneal swelling from the high pressure - this is an emergency.
Elevated IOP. Most commonly causes no symptoms until significant damage is done (glaucoma). This is why routine eye exams are necessary. A sudden acute rise in pressure such as from angle closure can cause pain, smoky vision with halos around lights and even nausea and vomiting, but this is much less common than chronic open angle glaucoma with no symptoms.