Reconcilation is the process of comparing the data after it is transferred to the BW system with the source system. You can use SCN search option for more documents.
Here is one.
Reconciliation will help you to compare the data between source and target systems/objects. It can be very handy to check loss/error occurred due to loading failure(s).
I have posted a document on SAP BW/BI Testing, it also highlights the reconciliation and different methods used for reconciling the data.
There is no Standard Reconciliation methods, It all depends upon your feasibility and your Logic to compare your data target with your source Data.
you can follow as your wish but make sure you go from the Top -Down approach or Bottom -Up approach and compare between multiple steps whether your logic correctly apopulation on each step.
Reconciliation is comparison of data from R/3 system.
There is specific method to comapre the data from R/3 system, it depends on Flow and logic that is used in the design.
You can follow from Top - Down approach.
First check the data in Final target if it is not matching then you can check in the underlying Infoproviders.