The usual method used to knock-out the gene and then observe the detectable phenotype is:
In this experiment, a recombinant vector transporting selectable gene with the gene of interest is employed. This is done to transform the yeast cells and grown under suitable conditions.
In contrast, the yeast cells that have introduced the marker gene will be chosen. A lot of markers will have the gene of interest disturbed by the selectable gene.
Thus, the gene function would be determined by the phenotypes produced.