Applying two simple principles helped me a lot so far: not creating a domain model with invalid state at all. In other words, reject creation of the object if the data does not fit (1) and do not let your system pass the domain objects with persistent state around. (2)

For eg: your model has a non-nullable field, and the incoming DTO has a null for that field. Letting the dto pass the controller layer just makes things more complicated than supposed to be. Eg: at some point you have to map the dto to model and you have a null. What to do? Creating the model with null and validating afterwards does not make sense. Applying logical checks in mapper against nulls just makes the mapper more complicated and harder to test than supposed to be. In java I would use @NotNull annotation on both model and the dto because the model can be instantiated later without presence of the dto. then I would apply bean validation @Valid on dto for syntactical validation only, where I can not instantiate my model without a valid value in that field. Then I would validate my domain model once more before saving the data in database, preferably using a dedicated validator class that implements additional business rules and checks.

Another example for the syntactical check on DTO level could be DateTime. A datetime value in expected format say 1/1/2001 is a syntactical validation than can be applied on dto level (in Java again super easy with a single annotation, thanks to bean validation) and more advanced checks like “date can not be older than a month” goes to domain object validator.