<p>I'm getting ready to start a new project and I've been researching the entity framework. My question is what is the best strategy for validating the entities? Other projects I've worked on have used attributes for most of the validation, but obviously this is not possible in the entity framework. Is the only way to do this by handling the partial methods in the property setters? All advice is much appreciated.</p>

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.