How to identify if a concept should be represented as a entity or as a value obj

I am trying to find out what question one must ask during the modelling process to detect if a concept should be modeled as a entity or as a value object.

My current understanding is the below

If in our domain a car engine can be swapped out and put in another car then it has existence of its own (Aggregation) and hence cannot be a Value Object ... However in our domain if the engine cannot be swapped out and hence makes no sense without a car then it can be modeled as a Value Object (Composition). Does this make any sense ? 

Please let me know what you think 

Thanks



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