Some time ago, I read in a newspaper--though I can't recall the details--a list of (mostly artificial) sweeteners and a list comparing how many times sweeter each was than sugar. How is relative sweetness quantified? Is there a standard method?
Response of the gustatory receptor neurons can be measured using standard electrophysiological techniques, under different conditions(exposure to substances). For example see this article.
However, it is difficult to say if a person feels sweetness or not or how the substance produces complex taste perceptions in some people.