Angel has the right idea.
You can make some educated guesses on how your game will run on different systems (using benchmark test and system specs) but, in order to test on different systems you have to physically test on different systems. Emulation will not work.

The "big company" (i.e. Microsoft, EA) way to do this is to create a lab with dozens or even hundreds of computers. All of them set up differently.

The "small company" (a few dozen people) way is to hire a 3rd party to test in their lab.

The "indie" way is to create a demo of your game (something that plays itself, think 3DMark) and give it out for free. Have a auto-report feature that uploads the results to a website. People will download your demo and run it just to see how they compare to the rest of the world.