Put butyl chloride and water in the same flask, shake, and let stand overnight then pour off the butyl chloride.
If you want a reproducible result then follow above instructions and weigh the amount of butyl chloride added to the known amount of water and after overnight equilibration, remove and weigh the butyl chloride that did not dissolve into the water. Of course, degas the water before adding the butyl chloride and measure the temperature of the saturated water for your records.
Did the above responses satisfy your query? I have no experience with this binary system but in general, after the 'equilibration' both phases will be mixed, i.e. the organic phase will be saturated with water and vice versa. This process may be accompanied by change in volume. One way to do the observations quantitatively is in a calibrated centrifuge tube.