I found this related question: What is the CR/XP-Character Level relationship for opponents? but it was asked before the DMG was released and it does not appear to have been updated to include information from the DMG. I could be mistaken but from my reading through the DMG I don't know that this information is contained therein (though the related question and information from the MM seem to indicate there should be something). The section on creating monsters does have a way of calculating CR.
Is it intended for the DM to use the custom monster CR calculations to calculate CR for NPCs? Is the information needed to calculate CR for NPCs with class levels in the DMG or is this left as an exercise for the DM?