How are geneticists able to isolate gene functions?
@LanceLafontaine suggested a few ways above, but I would recommend reading this section
in Alberts' Molecular Biology of the Cell, which gives a more descriptive overview of some approaches, but even that is not exhaustive.

An extremely simplistic and also not exhaustive summary:


observe mutants, see what processes are disrupted, and map these back to particular mutations
mutate a gene and see what processes are disrupted
biochemical studies to see, for example, which proteins (which are gene products) interact in vivo and things like where they're localized, which may hint at function.
analyze structure of proteins, see how similar they are to other proteins with known function, and infer that the protein of interest probably has a similar function


EDIT: The link to Alberts is to the 2002 edition (which is free online; the latest edition of the textbook costs around $200), so things have come a long way since then, but the underlying approaches are still in use.