You need to know the basic vocabulary. When someone says that two genomes are 95% identical, that has a very precise meaning. You need to know genomes are made of chromosomes, containing long strings built with four letters, that amino-acids are made of groups of three of these letters, and that the possible mutations include deletions and insertions as well as substitutions. Once you know this, the claim is a mathematical statement, and you can test it in many ways.