As mentioned by others, the best way is to get the a beta out there and gather community feedback to get reliable results.
I'd also like to suggest the use of ESXi. It's a Virtual Machine solution from VMWare. Don't be put off because it's a VM solution. ESX is a lot more low level.
This would probably be ideal to you since you have one machine. You can test different operating systems, hardware setups (bar the graphics hardware, ESXi would use whatever you have installed on your machine).
It's basically its own very slimmed down operating system that you can install other operating systems on top of. We use it at work for compatibility among a great deal of OS's. It's very fast and reliable, almost as responsive as a vanilla machine.
I know hardware requirements is what you're looking for and ESXi has a very nice feature where you can limit the amount of RAM, CPU or disk space usage by the operating system and could potentially emulate how your game would run on anything from a low end CPU or low memory, to top-end CPU or memory use (limited by your actual hardware).
So if you had quite a high spec PC you could install ESXi on it, install whatever operating systems you wanted to and play around with the CPU and Memory usage (I believe you can even modify the number of cores the OS uses as well!)
Here's a link to it: http://www.vmware.com/products/vsphere-hypervisor/index.html
Best thing about it? It's 100% free!