If things start to look not as clear as it was before... Then thats a sign of needing new glasses. You can tell easily by putting on ur glasses in the morning after a good night sleep.. Because your eyes are relaxed.... But another reason that you might not be able to see extremely clearly... Is pby because your eyes are tired... You need to relax after a while of using glasses.. Don't concentrate of one area for over 10 minutes... Esp. Computer.. Take ur eyes off it.. And look at something else... Like green trees or something... Then you'll feel better... Because if your eyes are tired...... You won't be able to see all that clear... Your eyes are fluctuating. Good luck!! :)