In Mathematica,some function is classified as kernel function,such as Sin,UnitStep or Abs and so on.We can use System`Private`HasAnyCodesQ to judge it is kernel function or not,I have metioned it in this post this answer recently.If the function is not a kernel function.Since they all are not,we can use GeneralUtilities`PrintDefinitions to get some some information of what you want.
<<GeneralUtilities`;GeneralUtilities`PrintDefinitions[FindGeneratingFunction]
http://o8aucf9ny.bkt.clouddn.com/2016-12-13-07-03-03.png
We can see the work fuction is call a name of findGeneratingFunctionParser,click on it.You will get its page of definition,then you will see findGeneratingFunctionDispatcher,findGeneratingFunctionPoints and iFunctionSpace.We can know it find generation function in 5 case:
http://o8aucf9ny.bkt.clouddn.com/2016-12-13-07-09-55.png
You can use same method to find some information for FindSequenceFunction.Those information is more useful to you maybe.It's very clear.