How would you find out what function a gene has in humans?

If you only know about Mendelian genetics, you could suggest searching the genome of relatives to a person known to have the gene to see if those with the gene share traits. This would be a good time to mention a technique if you know it e.g. PCR. However, it may not be possible to isolate gene function this way because: the gene would need to be active, any trait it contributed to would need to be single locus to really see an effect, it could produce a hidden condition that isn’t apparent in a pedigree, the gene could be essential and thus present in all relatives, the sample size is so low making spurious correlations very likely, this would only be a correlation study and not an experiment that determined cause and effect (experiments requiring human breeding are, at best, inconvenient).
If you are more familiar with genetics you may have other ideas; perhaps a cross-species survey looking for the gene in other species to judge its age and specificity to human life, or knock-out genetics with a closely related species possessing the gene to see if its presence is vital or has a direct effect. The gene could be added to a bacterial genome using plasmids to see which protein is produced. No matter what your answer is, the important part of this question is to take a practical and self-critical approach which plays to your individual knowledge base.