Q. A few of our employees have added their spouses to our health
A. If you have reason to suspect fraud (and the co-worker comments are reason enough), you can legally ask employees to verify their marital status. You can ask for any documentation that establishes a marriage, such as a marriage certificate, titles to a car or home in the spouses' names or a jointly filed tax return. Give the employee a choice. Some employers ask employees to complete “marriage verification” forms, which essentially are affidavits.
Final point: If your state still recognizes common-law marriages, don't treat a couple bound to marriage by common law any differently than a couple with a “properly” solemnized marriage.