What is the best approach to check if a lookup field is null in apex?

    rec.lookupField__c == null;         // OR
    rec.lookupField__c == '';           // OR 
    String.isBlank(rec.lookupField__c ) // ?


I apologize if this is the same as http://salesforce.stackexchange.com/questions/76815/how-to-check-in-apex-if-a-text-field-is-blank. I posted this anyway because I need confirmation on the approach specifically for a lookup field. The linked question talks only about text field. I am not sure if it is completely safe to consider a lookup field as a text field in this scenario.