This will depend on the school. Some schools have a form, others simply expect an e-mail reply. If you don't recall seeing a form, send an e-mail response* and include a query as to whether you need to do anything else to formally accept/reject the offer. Even if there is a form, you may also want to send an e-mail to the chair of the graduate admissions committee, as the form may go directly to the graduate school, not to the department.
*the e-mail should be sent to either the chair of the graduate admissions committee or to the department, not to a random faculty member with whom you have had correspondence.