This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
If equal volumes of a solution of potassa and of a concentrated solution of picric acid are mixed, picrate of potassium is precipitated, which, on warming, dissolves to a transparent, orange-red liquid. If glucose be added to this, the liquid becomes purple, and almost black. Cane-sugar does not yield this reaction unless it is inverted, in which case the change of color takes place at once. The reaction takes place only in alkaline solution, and is sharp enough (according to Johnston) to recognize 1.5 gm. glucose in 10 liters of water.
Many other tests have been proposed, but the above are quite available and give good results. Tannin and similar compounds which are likely to interfere are previously removed, either by solvents or precipitants (mineral acids).