taste.
you can also use a whip to mix everything over a steam bath. the trick is to never ever stop stirring while you add all the ingredients. there should be a nice consistency to the sauce. if it is too liquid, it is not going to taste good.
hollandaise sauce has a tendency to turn. if it happens, throw it away and start over.
hollandaise sauce does not keep. serve right away, and dispose of the rest quickly. 