<section ng-init="tab = 1">
<ul class="nav nav-pills">
<li ng-click="tab = 1" ng-class="{active:tab === 1}"><a href="#">Descr</a></li>
<li ng-click="tab = 2" ng-class="{active:tab === 2}"><a href="#">Specs</a></li>
<li ng-click="tab = 3" ng-class="{active:tab === 3}"><a href="#">Reviews</a></li>
</ul>
{{tab}} <!-- Вот здесь Вы сами и выводите номер таба -->
</section>