Prove it.
http://statesidelegal.org/citizenship-children-born-servicemembers-overseas
3.The child’s parents were married before the child was born. One parent is U.S. citizen and the other is a noncitizen. Before the child’s birth the U.S. citizen parent lived for at least five years in the U.S. At least two of those years were when the U.S. citizen parent was older than 14.
Ted Cruz’ birth fits all those criteria.