Theoretical papers are normally concerned with abstract issues around a certain topic. They differ from Empirical studies as they don't usually contain results or new, original research or data. They are still reliable and usually peer reviewed but a lecturer may sometimes request that you instead use Empirical/Research articles.
Rider University has a useful guide to the different types of articles and in it they suggest you look out for terms such as; concepts, conceptual, framework, model, theoretical foundation and perspective, when trying to identify a theoretical paper.
