1. Checking the Screen or Monitor
The screen of your computer or monitor has pixels. These pixels have three colors: Red, green and blue. Pixels can often be damaged or stop functioning properly displaying only one color or just being black.
There is an online tool to check for bad pixels called "CheckPixels" The test at CheckPixels will display your three main colors mentioned above and if a pixel is dead, it will show as a black spot and if a pixel is glowing a different color ( Malfunctioning) you will see that too.
2. Checking the Keyboard
The keyboard might not seem important as a hardware component, but it is used in almost all tasks and taken for granted. For laptop users, the keyboard is very important. If one has to send the laptop away for repair the entire computer must go with it. There is also an online test tool available to test all your keyboard keys called "KeyboardTester".
3. Checking the Disk Drive (HDD & SSD)
Your HDD / SSD has your operating system and most of your applications and files. Having your HDD / SDD completely healthy is very important. This is most imperative when dealing with a used computer since you have no awareness of the previous history or care of the used computer and its HDD / SDD.
Different tools are available to check the condition and health of the HDD / SDD.
HDSentinel is one you can use for your HDD. The left hand side of the screen will display drives connected the computer. Health is the main parameter to look for. HD Sentinel will explain the meaning of the health percentages and the steps that you should take based on the results. As an example, you should not throw out your HDD just because of a few bad disk sectors and IO errors.
SSDlife is another tool specifically for an SSD. It also shows health in percentages and the expected life of the drive. Similar to hard disks, bad sectors can occur in SSDs.
4. Checking the Processing Units (CPU & GPU)
The main components that do all the processing are the Central Processing Unit (CPU) and the Graphics Processing Unit (GPU). These are the components that allow you to run your office applications and your games. These are also essential components to ensure are in perfect working order. So it is necessary that these two components should be in perfect condition. For testing both, we use CPU speed test
For testing your GPU, the tool.Base Mark Web 3.0 works just fine.
5. Checking the Random Access Memory (RAM)
RAM is the one essential component that has the most immediate impact on your computers performance. MemTest from HCI Design is a great tool to check your unused memory for errors. Identifying and fixing errors with your RAM is important. Errors in RAM cause the infamous "blue screen of death". Boot problems can also be attributed to RAM errors.
Besides the hardware component tests above, you should also check your Ethernet ports and connection, wireless connection from the wireless card (internal or external), HDMI, DVI and USB connections using available cabling and devices.