The answer is multidimensional and depends on the type of fluid that is being evaluated and for what purpose.
For example, a chemist could determine the composition of a fluid, such as its molarity or ionic concentration. However, if the fluid under study is blood then the answer would be a hematologist. A hematologist is concerned with the study, diagnosis, treatment, and prevention of illness related to issues within the blood. If it is water from a river that is being studied, then the answer would be a hydrologist, ecologist, or even a biologist. If the fluid of interest is lava or magma, then the answer would be a geologist. An oceanologist may study of the composition of sea water in order to understand the causes of a decreasing population of organisms.
These are just a few examples.