D-glucose & D-fructose can be differentiated by :

Br₂​/H_2​O being a mild oxidizing agent can only oxidize aldose to carboxylic acids, but cannot oxidize ketose.

As glucose contains aldehyde group, it gets oxidized and gets converted into a carboxylic acid and it can be differentiated from fructose which has a ketone group.

$\mathrm{D}-\mathrm{glucose} \underset{{\mathrm{H}}_2\mathrm{O}}{\overset{{\mathrm{Br}}_2}{\to }} \mathrm{D}-\mathrm{gluconic} \mathrm{acid}$
D-Fructose → No reaction.