One of the most challenging aspects of implementing peer-to-peer approaches is making sure that what gets implemented has integrity to what peer roles are intended to be (and stays that way). Although imperfect, there are a few 'fidelity measures' out there that can provide some guidelines. Two of them include:
Toolkit for Evaluating Peer Respites: This tool kit was just released in November 2014 and was developed by Laysha Ostrow (of Live & Learn, Inc.) and Bevin Croft 
(of the Human Services Research Institute) through funding from the National Empowerment Center, the National Mental Health Consumers' Clearninghouse, and the Foundation for Excellence in Mental Health Care. As the name suggests, it is focused on evaluation of peer respites and was developed, in part, through extensive interview and research processes with existing respite houses. Although it's intended for respites, it is a tool that can be useful for anyone involved in any aspect of this work. The toolkit is available from the NEC's website.
The Fidelity Assessment Common Ingredients Tool (FACIT): The FACIT tool was developed through the Substance Abuse and Mental Health Services Administration. It is a part of the 'Consumer 
Operated Services Evidence-Based Practice Kit,' that contains a number of elements ('Training Frontline Staff,' 'Using Multimedia,' etc.) with contributions from Shery Mead, Pat Deegan, Jean Campbell, Darby Penney and a variety of others. The FACIT tool offers several pieces, including the values of measuring fidelity, items that should be considered, and so on. Much like the peer respite tool, FACIT is designed primarily for evaluation of supports that are fully peer-to-peer (peer-to-peer community centers, etc.), but still may contain some useful elements to consider even in traditional environments. You can review the full kit in the SAMHSA store.