Can a dextrose / caffeine powder be (mostly) separated by dissolving in water then filtering the water?
Dextrose is readily soluble in water (1-10 parts solvent to 1 part solute). Caffeine anhydrous is sparingly soluble in water (30-100 water : 1 caffeine).
I'm assuming water would dissolve nearly all the dextrose (and a bit of caffeine), and upon reaching saturation, the caffeine will remain out of solution in solid form and easily filtered.
Repeating the process would refine (increase) the quantity of caffeine vs. dextrose in the remaining filtered solids.