If you don’t have a sugar thermometer, test a little sugar syrup by dropping it into cold water to see if it’s ready.
Sugar goes through several stages as it boils: thread, soft ball, firm ball, hard ball, soft crack and hard crack. As the liquid boils away, the temperature increases and the concentration of sugar become greater. Each stage describe wahat its consistency will be when dropped into cold water.
Fudge needs the soft boil stage, while marshmallows are cooked to the hard-ball stage. Ideally, you should use both the cold-water test and a sugar thermometer for the most accurate result.