For Microsoft created fields, entities, web resources, etc. the schema name won't have a prefix or it'll be msdyn_ (e.g. industrycode, name). Anything that was built out in the default solution would have the prefix of new_. You could try and search out anything that has that prefix. Plugins should be able to be identified as non-Microsoft. The Microsoft ones would start with Microsoft or ActivityFeed.
This might not be all encompassing but should get you moving towards your goal.
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