Use extra caution when boiling mixtures that contain dairy products. These candies have a tendency to stick to the bottom of the pan, and if boiled too hard over too-high heat, they will scorch and may even curdle.
If a thermometer is not available, test candy mixtures by using the cold water test. Working quickly, drop a few drops of the hot mixture from a spoon into a cup of very cold (but not icy) water. Working in the water, use your fingers to form the drops of candy into a ball. Remove the ball from the water. The firmness of the ball will indicate the temperature of the mixture. At the "soft-ball stage" the candy instantly flattens and runs between your fingers. At the "firm-ball stage" the candy is firm enough to hold its shape, but quickly flattens at room temperature. At "hard-ball stage" it can be deformed by pressure, but it doesn't flatten until pressed. At "soft-crack stage" the candy no longer forms a ball, it separates into hard, but pliable and elastic threads. At "hard-crack stage" it separates into hard, brittle threads that snap easily.
If the mixture has not reached the correct stage, continue cooking it for another 2 to 3 minutes. Then, quickly retest the candy, using fresh water and a clean spoon for the test. Continue cooking and retesting till the desired stage is reached.