How can you tell when you have an elevated eye pressure?

Cant. Thats the problem. I rare cases, if the pressure gets very high, it will hurt. But thats not typical. So you need to get the pressure checked on a periodic basis.
Usually none. If you have open-angle glaucoma there are usually no sensations to tell you this. In patients with closed-angle glaucoma there is an acute rise to dangerous levels and this is accompanied by visual changes and eye aching. This is an emergency.