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

    lookupField__c == null
OR

    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 , posted this anyway because need confirmation on the approach specifically for a lookup field. The mentioned SFSE 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.

Please let me know your thoughts. Thanks in advance.