By taste.  Subjects are asked to report whether taste solutions containing
various concentrations of the "test" sweetener are more or less sweet than a
standard concentration of sucrose.  There will be a range of concentrations
where most subjects find the "test" sweetener and standard to be equally
sweet.  This is used to calculate the relative potency.

The typical figure for saccharin is, I believe, about 300x sweeter than
sucrose by molarity.  That is, a 0.5 M sucrose solution (about 17% sucrose)
is roughly equivalent to 1.67 mM sodium saccharin (0.034%).

Mike Tordoff
Monell Chemical Senses Center
Philadelphia, PA 19104
www.monell.org