I don’t think there is a need for a thermometer when one is “boiling down” cane juice for making molasses ……. or Sugar Maple sap for making Maple syrup.
One just keeps “boiling” the cane juice or maple sap until it is reduced down to the desired “color” and/or “viscosity” …….. then you “jar” it (molasses) or “bottle” it (syrup).
I’ve tapped the trees and made Maple Syrup several times ….. and when I was 3 ta 5 years old ..... my Dad raised a patch of Sugar Cane and I remember watching them make molasses.
This is how they “crushed” the juice out of the cane.
And then the juice was poured into a great big “evaporator” pan with a fire underneath.
And I still don't like the taste of molasses to this very day ...... because it was those molasses I had to eat iffen I wanted something "sweetened".
Sugar was "rationed" and mom couldn't buy any.