Maybe you're looking for an answer of how we know how much carbon there is in a particular bloom or billet.

When I smelt iron sand into a bloom, as it's called, there's varying degrees of carbon infused all over and through the bloom. I chop up the bloom and spark test the different pieces. Depending on how much carbon there is, the spark will look different starting from a short dull streak and gradually adding more and more forks in the spark until it looks like a mini firework and then back to long dull sparks and then short ones again.

In the early days, smiths and foundrymen probably didn't explicitly know it's about carbon content, but relied on feel and observation, the sparkly bloom makes the stronger sword etc.