is there a way to list all the fields both standard and custom for a given object.
tooling api - customfield gives only list of custom fields but not the standard fields.
You can just use describes.
List<SObjectField> allFields = SObjectType.My_Object__c.fields.getMap().values();
For API services, you can call either the REST describe call or SOAP describe call.
The Developer Console actually has a third option. Click on File > Open Resource, and type in the object's name, such as