As the syrup heats up it goes through different stages that have distinctive names in candy making, for example "soft ball", "hard crack" or "clear liquid". For toffee we are aiming for the "hard crack" stage. For testing you will need a bowl of water and a dry spoon. Once the syrup starts to develop an amber color you can start testing the temperature. Remove the saucepan from heat and then drop a little syrup using a dry spoon into the water. Use your fingers to pull the ball of syrup into a strand. After the strand solidifies, it should snap when you bend it -- this means you've reached the right temperature and you can add baking soda.