Without a form, you can fix this by check the value of the models in your controller, returning an error if both are false. You could also go ahead and set one true by default.
But to answer your question, you can do something similar to $scope.myForm.gender.$invalid all you have to do is wrap your input tags in a form with the name myForm. So, it would like:
<form name="myForm">
<input type="radio" name="gender" ng-model="male">
<input type="radio" name="gender" ng-model="female">
</form>
Then, $scope.myForm would be able to give you certain properties, like $isPristine and properties for each input field.
Either of these ways will work though, so I help that helps!