How do you determine a margin without some form of a baseline?

You read the operator’s handbook which will give you that information.

In a certified aircraft the manufacturer has already done the test flying

Your airplane can also be in the "experimental" category, which generally means "homebuilt" so the stall speeds and characteristics can only be determined through test flight. I made the decision to empirically demonstrate stall speeds on the very first flight of my homebuilt, before attempting to land, so I knew for sure what they were.

I also have a homebuilt, and went and found out my stall speed.

But most homebuilts will still have a stall speed for planes built to plan.

In any case, this is an article about a Cessna. Where the stall speed is in the POH and also marked on the ASI.