How do I evaluate complimentary purchase template engine in predictionIO?

Question

I use predictionIO, build and trained a model for complimentary purchase template engine: https://github.com/PredictionIO/template-scala-parallel-complementarypurchase

Now I need to evaluate the results. But the evaluation part in predictionio docs is missing. Could someone please give me an advice, which metrics should I use and which evaluation techniques?

Thanks a lot.