How do medical researchers find out if individuals of both sexes have similar symptoms or react the same to treatment?

In response to the treatment side of this question. Prior to clinical trials, the pharmacokinetics (what the body does to the compound) of putative drugs are tested in in mice and other small animals. These preliminary studies help the investigator to determine which enzymatic pathways are involved in metabolism of this drug. If data acquired from previous clinical studies suggest ethnic or gender based differences in drug response related to the metabolism of this particular enzyme this should be taken into account when healthy volunteers are recruited for phase I trials.