I suppose that concern looms ever larger with today's single-parent families. In the past, you pretty much knew (to some degree of certainty) who your date's parents were... you met them in person. However, the blood tests required as part of marriage license process in the past were never a protection against extended-family nookie. Mostly, the tests were to check for syphilis and other STDs.
Now that DNA tests are available, that *could* be used to check for dangerous similarities. IIRC, most states dropped the blood test requirements before DNA tests were accepted as legal evidence by the courts.
Thank you for the DNA comment. Makes perfect sense for ALL couples planning to get married and start a family to invest a little time and money to be sure that their children would not face inherited genetic problems.