I have not actually used the Entity framework before but a quick search indicates that you have several options.
1) Validate at another layer in your application
Always an option, I just thought I would throw it out there explicitly.
2) Hook into the OnChanged events of the Entity then perform validation
Likely brittle and would become confusing/slow after if you have many different properties things that can change for each entity.
3) Implement partial methods to validate property changes
According to this post and this walkthrough there are partial methods available for validation. This seems like your best option as it is not very intrusive and you can selectively implement the validation you want.
I hope that helps. Good luck.
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