1. Make sure that everything is plugged in. It's entirely possible that a cord could just slip out from the outlet while you're working. If power is present to the monitor and other peripherals, yet there is no power to your computer, there is likely something wrong with your power supply.
2. Hit the power button. The most obvious clue is that the system will do absolutely nothing when you hit the power button. If there is no sound and no monitor activity of any kind, the power supply has probably died. While this can also be caused by a faulty switch, it is usually the result of a burned-out power supply.
3. Look at when your computer boots up. Noticeable changes in how long it takes for your computer to boot up and shutdown, as well as spontaneous rebooting, could be a sign that something is wrong.
4. Check for beeps. If the system makes rapid, short beeps repeatedly and does not boot when you try to get it to, this could be linked to the power supply.
5. Observe any computer failures. If there are system startup failures or lockups, memory errors, HDD file system corruption or USB power issues, this is often directly related to your power supply.
6. Check the fan in your computer. If the fan in your computer is failing to spin, it could lead to overheating and smoke in the system, all of which can cause a failed power supply.