How to know when to use lowercase attribute when updating via Web API

Hi,
I am trying to update Lookup attributes of a quote via the Web API.
I understand that in case of custom objects I need to use the camelcase notation, while in case of system should use lowercase.
But how can I distinguish between system lookups that come from dynamics library (like msdyn_account) and those that are "real custom"?
To get the attributes info (of a quote, for example), I am calling /api/data/v9.0/EntityDefinitions(LogicalName='quote')?expand=Attributes
In the case of my custom attributes and those coming from msdyn_* package I am getting
IsCustomAttribute: true
So how can I tell?

Thanks,
Roy