You may notice the temperature gauge in the middle of this picture. This is used to see if the syrup is ready for the next step. Richard says that this is not always precise. So he takes a steel/aluminum ladle with holes in it and watches the syrup drip. If the syrup drips off in pieces and is thick then it is ready for the next step.
If the syrup is dripping off the ladle very fast and one drip at a time, then it has to be put back into the evaporator in order for it to become thicker and ready for the next step.