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.