You need to measure the gravity (density) using a Hydrometer. You need to measure it before the yeast goes in and then at regular intervals. The initial measurement tells you how much sugar is dissolved in the wort. Water is 1000 and you will usually have the gravity measure at 1025-1030, for example, when the sugars have been formed in the wort. Once you add the yeast the gravity will reduce down to around 1005 over time. The lower the better meaning your yeast has done a better job of forming alcohol and there is less sugar dissolved. The alcohol content is the difference between the initial gravity and the final gravity readings.
P.S. The scummy stuff is the activated yeast doing its thing.