Computer monitors are designed to work properly for several years. Proper maintenance and good handling care can keep them in good working condition for a longer time. Some people use the same monitor with multiples systems for many years. This does not mean that monitors are invincible, though. Whether it's from a defective part or power surge, they go bad eventually, and you'll probably know it when it happens.
Look for random discoloration when your computer monitor is on. Look for spots of black or other colors that shouldn't be there. Look also for vertical or horizontal lines across your monitor screen that shouldn't be there.
Time how long it takes for your computer monitor to come out of sleep mode. It shouldn't take more than 5 or 10 seconds for the screen to be fully operational when taken out of sleep mode or turned on after being turned off. If the screen is not operating as quickly as it should, it could be a sign it's getting old.
Notice how bright your monitor screen is. If it's not as bright as it should be, try changing the contrast and brighten settings on the monitor. They'll most likely be located either on the monitor or on the keyboard. If you don't see a difference, the monitor is weakening.
Verify that your computer monitor comes on out of sleep mode. When you don't use your computer monitor for a while, it enters into sleep mode to conserve energy. If you move your mouse around and the monitor doesn't come out of sleep mode, the computer monitor may be going bad. If you need to turn your monitor off and back on in order to get the monitor our of sleep mode, your computer monitor may be defective.
Check for monitor lagging between the time you move your mouse or type something on your computer and the time these changes appear on your monitor. If you know that your computer does not lag, but when you move the mouse or type and the monitor takes a long time to respond, your monitor may be the culprit.