I have seen a lot of articles about common ancestors. But I didn't find any perfect articles that said about the evidence of common ancestor.
The most definitive conclusions about the common ancestor are made by comparing genetic sequences of the organisms, for which well developed mathematical/computational methods exist, see, e.g., Inferring Phylogenies or An introduction to molecular evolution and phylogenetics.
For distant species the genomes are rarely directly comparable, so that some ambiguities can exist. Therefore the existing tree of life and species classification are based on comparison of the ribosomal RNA, which is believed to be rather conserved:
rRNA is one of only a few gene products present in all cells. For this reason, genes that encode the rRNA (rDNA) are sequenced to identify an organism's taxonomic group, calculate related groups, and estimate rates of species divergence. As a result, many thousands of rRNA sequences are known and stored in specialized databases such as RDP-II and SILVA.
Note that this still leaves out the viruses, which are acellular and therefore do not have theor own rRNA.