WARNING: Doing what is described in this tale tail does have the potential to FRY YOUR SYSTEM if not done correctly. Caveat emptor!
Sometimes in diagnostics it is hard to re-create the issue described by the customer, so you find yourself having to push different components to their limits. Knowing which is the best tool to stress a particular component is important, so here is a list of my favorite free tools:
Black Magic Disk: This is advertised as a tool to measure disk performance for high quality videos, but to do this it will perform a write test on your hard drive, and if your Mac has a mechanical hard drive, this will help you to determine mechanical failure.
GpuTest: This is a very lightweight and simple GPU stress test. If you think your onboard graphics processor is glitchy and is causing weird graphical issues, then stress test with GpuTest. This tool works well and is simple to use.
Yes: The yes command is built into your Mac via a Terminal command. This command runs each core of your CPU at 100%. This is a great tool to see if you CPU is having issues. More on the yes command can be found here.
Just for science, I decided to run all three to see what would happen. I used an app called HWMonitor, and Activity Monitor (a built-in app; Applications > Utilities) to watch sensor temps and make sure I wasn’t going to overdo it and melt the Mac into a puddle of aluminum and circuit board. I was able to push all cores on the Mac to around 200 degrees Fahrenheit and the GPU peaked at around 180, all while writing large data blocks to the hard drive.
The computer was extremely unresponsive to while trying to run any other apps while under this stress test. Even opening a Safari window was painfully slow. After the test I rebooted the machine and immediately started to run diagnostic tools across the whole spectrum to see if I had started a slow death of the components and, to my surprise, the machine withstood a 2 hour onslaught of pure technical agony and was fine. That certainly says something about the quality of hardware in my 2011 MacBook Pro i7 15 inch.