I would say that Browserstack *isn’t* the **best** way, though, which is what the question is asking!
Browserstack is convenient, but it’s nowhere close to booting up in a proper testing environment. I use VMWare Fusion and have XP, W7 and W8 installed. But even then, the text rendering isn’t exactly like it will be while viewing in the real thing.
I would argue that the best way to test would be to have a dedicated windows machine with multiple drives, each with their own OS installed.
There are several possible combinations one can make. I always use what I think comes with the reality my users (or the website’s users), sometimes even going to Cafe to see eral live how things really work.
I personally use Sauce for Mac – https://saucelabs.com/ — @thedoc if you haven’t checked it out, you should, not a dedicated machine, but still very slick. You can also spin up Sauce Connect to test locally.