Following the announcement for Stadia at Google's first-ever Stadia Connect conference which it streamed live on YouTube through the official Stadia channel, Google put up a speed test tool that allowed users to test their internet connection speed to see just how good of an experience they might have with the service if they were to use it. It works, but it also seems pretty inaccurate and that's ok.
While having knowledge of this information is good to know, Google has really given you all the information you need to decide whether Stadia will be able to provide you with an experience that you can be happy with, or rather proud of. It listed all of the requirements for 4K streaming resolution which is a mere 35Mbps. Since most ISPs offer speeds well above that many people are already in the clear.
Still, if you're unsure of what your internet speeds are and you want to reassure yourself that the service will work just fine for you if you subscribe to it, Google has you covered on that front with a tool of its own that will give you the detected speed and tell you whether or not it will support the Stadia service in 4K, which is the best possible resolution on offer for it.
If you use this tool to check your speeds, then it's safe to say that you probably already know of other tools which will allow you to figure out this information, but whether you do or not it's worth double checking with other tools anyways. The Stadia speed test which you can find at http://projectstream.google.com/speedtest doesn't seem to give you the best results every single time.
After checking speeds through this tool on multiple occasions over the course of five minutes or so, they came out with a surprisingly varying degree of results, none of which were really even close to the speeds that were actually coming through according to other tools, which were not only more accurate but also a lot more consistent, offering up just about the same download and upload speeds every time.
The moral is that while Google is attempting to help streamline the process for its potential users the tool in which it uses isn't doing a very good job at painting a picture of what's to be expected. Someone with a fast download and upload connection may be well over the requirements for 4K streaming but end up being told they may not get 4K if they use the Stadia speed test.
The reason this isn't really a big issue is that Google's tool isn't meant to be the end-all-be-all of internet speed tests, and it even tells you that the speed listed doesn't guarantee that you'll be able to use Stadia as multiple factors need to be considered, such as which device you're thinking of playing games on.
More to the point there are enough tools out there to tell you your speeds, which you can cross-reference with Google's very visibly listed requirements for what speed allows what quality for streaming resolution with Stadia.
Beyond that you could simply go about it the old method and just call your ISP and ask them what your average speeds are that you're supposed to be getting, and while they might not be able to tell you what you're actually getting they can tell you what you should be getting, so either way you'll end up knowing.
The bottom line is that the Stadia speed test tool is there for you if you need it, it's just not the only option and probably far from the most accurate one out there.