How do you differentiate blurred vision from a brain tumor and poor eyesight?

"See" doc in person: Your one-line question makes me concerned that you are frightened! Many things can cause blurred vision; tumors often have other associated symptoms, or specific kinds of vision problems. An exam by a physician can include not only testing your visual function, but also looking in your eyes for signs of other disease--not to mention a review of the rest of you for clues!