Yes, technically, you can make peanut brittle without a candy thermometer, but I don’t recommend it — especially if you aren’t very familiar or comfortable with making candy. As mentioned above, the caramel needs to reach the hard crack stage. If you don’t have a candy thermometer, you can use the cold water test. Drop a little of the hot candy, which should be a dark golden brown to amber color, into a glass or bowl of cold water. It should form brittle threads in the water that crack when you bend them.