My question is about ancestry/genealogy test. 

Suppose a genome service company, like 23andMe, extracts two people's genomic information. After that, how will the company test the relative relationship of two individuals? Will they use some algorithms to detect the identity-by-descent (IBD) segments? Could you please introduce me some effective but also efficient/simple algorithms for ancestry/genealogy test? So far, I only know detection by IBD.