How To Judge Doneness of Hollandaise
Egg-rich sauces and custards are fully cooked when the mixture is thickened and coats the back of a spoon. To determine doneness, dip a spoon in the mixture and run your finger across the back of the spoon. The cooked mixture will hold a firm line and not run down onto the stripe you've made. A mixture that's not fully cooked will be too thin to hold the line.