Re: Cousin Marriage
Its more of an interview although you can have DNA testing done too. A genetic counsellor takes a very detailed family history - marriages, intermarriages, diseases, troubles, ages of deaths, etc etc. Then they calculate odds and probabilities of having a child with any inherited disease or trouble.
If you go for the DNA test, they look for genetic maladies, missing or extra chromosomes and certain diseases or defects which are mapped and known. For example, Downs syndrome is "trisomy 21", meaning that there are three rather two of the chromosome 21.