CiviCRM Community Forums (archive)

You can use the search builder to find folks whose Date of Birth is NULL, but I can't think of a way to find someone without a relationship.  In theory, you could put everyone that DID have a spousal relationship into a smart group ("Has Spouse group"), then use the Search builder to say, "Date of Birth IS NULL and Group != "Has Spouse group", but Civi 4.3 has a bug where you can't search correctly for the absence of a group using Search Builder.

Oh - maybe you can use the Custom Search Include/Exclude group?  It's a bit convoluted, but you can create three groups - one that includes everyone, a second group that includes folks whose DOB is NULL, and a third group of folks who have a spousal relationship.  Then use the include/exclude search to include the first group and exclude the second and third groups.