Cooking custards requires careful temperature control. An instant-read thermometer is the most reliable way to judge when your custard has reached the proper temperature, but you can also dip a wooden spoon into the custard and run your finger down its back to judge doneness. (This old-fashioned method really does work.)
When the temperature is under 170 degrees, the custard will be thin and the line will not hold. When it’s in the range of 175 to 180 degrees, the custard will coat the spoon properly and the line will maintain neat edges—just right!