Identifying Things in the Problem Domain

Which technique is better? When might one be better than the other? When might it be appropriate to use both techniques?

How do you know if you have identified all the domain classes? How do you know if you have identified the right classes? What can you do to validate the classes you have identified?

How can you distinguish between classes and attributes? Sometimes nouns can be either a class or an attribute (for example Date). How can you know whether to make it a class or an attribute?

If you have one solution with a problem domain model and a colleague has a different one, can they both be correct? How can you tell if it is a correct model? How would you choose which might be a better solution?