I googled it, and the top result wanted to download/install a PuP.
https://www.systemrequirementslab.com/cyri <- Automatic detection
https://www.pcgamingwiki.com <- figure it out manually with the wealth of information
I just, uh, borrow them from a friend to see how they work on my rig, nothing else will give you a better representation, everything else will just be a guess.
A caveat to this is that sometimes your friend’s games run better since he/she removed the power-hungry annoying part that prevented you from borrowing it.
My friend also set up a custom accessibility control scheme so he could play games with his hook hand and issue voice commands via his parrot
YouTube. I know it sounds goofy, but often you can search something like “Baldur’s Gate 3 gtx 1060 6gb i7-4790K” (or whatever your specs are) and you will get tons of videos of people running it on their systems. If you happen to have common parts, you will not normally have trouble finding a benchmark for a rig very similar to yours for most games, but even with more niche hardware, you can usually find something helpful, even of it’s just like a similar GPU or another laptop with the same chipset, or whatever your case may be.
Beyond that, Steam’s hardware requirements on the store pages of games and pcgamingwiki are great resources.
I’d also say you can look on protondb–it’s for Linux gamers, so the results may or may not be applicable if you have a Windows system, but in most cases, if there’s a report that something runs well on Linux machine with the same hardware as you, it’s going to be very similar on Windows. The other way isn’t so applicable, though–just because something runs poorly on a Linux rig doesn’t necessarily mean it will also run poorly on Windows, as the problem could be with the compatability layer and not the hardware.
None of these are a perfectly elegant solution, but they are typically reliable enough.
Check Steam. It lists minimum system requirements on each games store page at the bottom.
As someone who released a game on Steam, I had no idea what to put in as the minimum requirements. I basically said “screw it” and put in the specs of the PC I started developing it on because I had no way to test it on anything else.
I always like when the recommended maximum requirements are clearly some devs high end rendering box with 256 GB of RAM and 4 Video Cards.
