In order to determine whether they were beneficial we'd first have to be more precise in what we mean by 'making you smarter' and then test that skill in tasks unrelated to the training that use the same faculty.

For example, if we hypothesise Chess benefits working memory we would do a series of unrelated working memory tests on a sample of individuals and look for correlations with their chess ability (for ease of testing, the same individuals as they progress across time would be best). Similarly, if we hypothesise Poker contribute to probability estimation ability we would test them on non-poker related statistics questions.