If I understand correctly, "gene-B" is a candidate, whose induction in cells contributes to a prolonged survival/proliferation of cells. In order to test whether the "gene-B" is "responsible" for the above phenotype(s), simply knockdown the expression of the gene in cells. If the  phenotype is completely lost or reduced, this observation would serve as a basis to use additional approaches to further provide support for the role of "gene-B" in the earlier phenotype.

Since it sounds like you are extending lifespan, you are not working in a cell culture system.  Instead you must be using a whole organism, yes?  yeast, C. elegans, or flies perhaps?   This sounds like an exam question.    You can knock-down or mutate gene B and ask whether Pathway A still extends lifespan in the absence of B.  If not then gene B is necessary for lifespan extension.  You can overexpress gene B by itself and if this extends lifespan then gene B is sufficient for extension on its own. 

Thank you Divaker and Gregory. I proposed the same method to test the function of gene-B through RNAi. However, due to the fact that gene-B is express in various tissue. it is hard to get a complete knockdown. So, my goal is to see if there are any alternative approach. I came up with one approach so far. I was thinking about making a synthetic gene-B. I will then inject gene-B into the organism and do a several assay. What do you guys think? Thanks