Not much to add to the title- what is the currently accepted "best" test of fluid intelligence? How was it determined to be the best?
You'll find an answer here:
As it mentions, Raven's Progressive Matrix is a popular option that has been shown to load highly on both g and Gf (fluid intelligence).
In terms of gold standard, it is probably preferable to have a battery of measures that are relevant to Gf and take some composite of these. For example, the WAIS performance subscales.
In general, a good measure of fluid intelligence would be inferred from its correlations with other measures. It should also have good reliability and other validity properties. Specifically, when administered in a large battery of cognitive ability measures, a factor analysis should indicate that the test loads highly on a factor that looks like fluid intelligence (i.e., that other measures of fluid intelligence load on). There's more to it than that, but perhaps a starting point.