I have a workflow which closes a case itself if the customer hasn't returned to us after certain time. After case is closed I want to populate a field with a certain value, but only if this field is empty. This field's type is picklist. I have a field update action defined but I cannot verify if there's already a value on this field or not. Please advise.
You can use ISBLANK function to verify if field contains any value or not.
Description : Determines if an expression has a value and returns TRUE if it does not. If it contains a value, this function returns FALSE.
Usages: ISBLANK(expression) and replace expression with the expression you want evaluated.
So in your case as this is picklist value your function will be like this ISBLANK(TEXT(fieldapiname))