It's important to remember all of this so far is done before the invention of the thermometer. We might think of the melting point of iron as a number, or the temp of a bit of metal as so many degrees. But that's a really modern idea in the grand scheme of things.
So how did we measure temperature without being able to measure temperature?
Colour.
If I have an object that's 1200°C. It will glow orange. Doesn't matter if it's steel, or glass or anything else.