To start using Maps API you have to load it with loader . Maps API is modular and you can select feature set to load . Mentioned before loader can accept a callback function to notify about the loader work. That is the scheme.
I don't think you can find somewhere in the documentation the feature list and use a feature use checking by name. I do not know if featureImplName is documented somewhere. Definitelly you can read already loaded feature list and obtain some value back -- but i do not see any use in it.
To check whether you components are loaded i suppose to use nokia.Features.load with event callback -- you do not need to know featureImplName in that case. I suppose feature listing is kind of the API extension support-
Please check the documentation update on Features , apparently it gets more clear:
}, onSuccess, onError);