In double-stranded DNA, every guanine base is complementary to the cytosine base in the opposite strand, so the GC-content of the two strands will be the same. count the number of G-C pairs in double helix. Because the GC-content throughout the genome differs between species, GC-content can offer a rough preliminary test of the identity of unknown DNA.
The GC-content of most species does tend to hover near 50%. However, coding regions of the genome have a tendency to contain a higher percentage of guanine and cytosine; these areas are called GC-rich, in contrast to areas of GC-content below 50%, which are called GC-poor. Thus, just as GC-content between species offers a rough test of species identity, testing the GC-content of a snippet of DNA from a known species can offer insight into whether that DNA may belong to a gene.