Workflow need to compare requestor and approver

Hi, 

I have a current workflow and need to add a step to check to see if the person who requested a purchase service is the approver.  Accounting laws don't permit these to be the same person, but I can't see to find a workflow step that would allow me to compare these fields.  I assume it's a condition but Condition only allows for a specific person or group and not to a field on the form.  I can choose the field on the form for the approver (budget manager), but then can't chose the requestor.  I've looked through the KB articles but not found anything helpful.

Thank you,

Greg