Evaluating Recommendation engines
The standard way to evaluate a recommendation engine is by using the RMSE (root mean square error) of the predicted values and the ground truth.

It is almost a SOP that, after finishing developing a recommendation engine, we will evaluate this engine by comparing its RMSE with other famous, common recommendation algorithms like SVD, tranditional CF, even RBM, etc.

Some terms mentioned above do not seem to be related with recommendation, but you can easily find on the internet how these techniques can be used in this topic.