How to check the account type on Opportunity record

 Hi,

On our Opportunity Layout we have two accounts fields, one is the normal account field and then we have another customized field that's called "Customer Account", this field is pouplated through a selector report. My question is, that is is there a way we can display a warning or error message if user tries to Close the opportunity when the Account type of the account selected in Customer Account is Partner? I was thinking of doing this via workflow but I dont see reference to Account Type field in expression builder. I hope my question makes sense..

Thanks,

Mo