His solution made him famous: Fehling's test

How does Fehling's test work?

The method devised by Fehling was based on a test solution of copper sulphate and Seignette's salt. The liquid under investigation was added to a measured amount of the blue test solution until the latter changed colour. The amount of liquid needed to cause the colour change could be used to determine the sugar quantity on the basis of a table drawn up by Fehling.

More details on how Fehling's test works