angularjs check box validation

I have 2 check boxes used like this

 <div class="group">
                        <label>Message to:</label>
                        <span class="checkbox-group">
                            <input type="checkbox"   name="mobile" ng-model="formData.mobile" />
                            <label for="mobile">Mobile</label>

                        </span>
                        <span class="checkbox-group">
                            <input type="checkbox"    name="email" ng-model="formData.email" />
                            <label for="email">Email</label>

                        </span>
                    </div>

i want to show a error message if atleast one of these checkboxes are not checked. i want to show the error message only after i press my submit button. How can i do this?