There are many ways to demonstrate the results of your SPM practices, to both internal and external stakeholders.
The SPI4 is a great reporting tool that allows you to document both your SPM practices and social outcomes. Many providers are creating SPM dashboard reports using the graphs generated by the SPI4. The SPM Resource Center has examples of SPM dashboards (see under Standard 2a "Tools and Templates").
To demonstrate your client protection practices, specifically, you can pursue a Client Protection Certification. Certification is conducted by an independent rating agency and indicates that the provider is doing everything it can to treat its clients well and protect them from harm.
Finally, you may also wish to have a social rating. Most social ratings also include some information on client outcomes, which you can use to demonstrate outcomes to external stakeholders such as funders.