Dynamics Portals : Portal contacts differentiation from MS CRM contacts

I am working in a project where i need to run a workflow only for the Portal users (contacts). Here i need to seperate the portal contacts out of all but what should be the condition to do this?

I think, since every portal contact will be under ownership of "SYSTEM", but i cant add filter of owner="SYSTEM" in workflow "if-condition" step.

Is there any other way I can filter this scenario.