You usually cannot unless you have had a recent and through eye exam.

In general, glaucoma often develops gradually and painlessly; there are no early warning signs. Therefore glaucoma is dangerous because it can gradually destroy your optic nerve and vision without your knowing. Although someone with the less common type of narrow or closed angle glaucoma may develop halos in their vision, headaches, redness in their eyes or even nausea and vomiting. But in general, glaucoma is very sneaky.