Offline evaluation of recommender systems
There is always a bias in offline testing in recommender systems. Real evaluation happens in A/B testing. But, that should not discourage one from doing offline testing. 

There is ongoing research on this topic using multi-armed bandits. I recommend reading offline testing procedures using reco-gym.

Following workshop talks about removing these biases and developing efficient offline estimators for recommender systems. They also conducted an online course on efficiently doing an offline evaluation of recommender systems.

You can also follow a video tutorial on this website on counterfactual evaluation