A common question with an unsatisfactory answer! Basically, measure Scope 1 and 2. 

Here are the big 4 I discuss:

B-Corp: https://www.bcorporation.net/en-us/

SBTi: https://sciencebasedtargets.org/resources/legacy/2020/07/SME-Frequently-Asked-Questions_July-2020.pdf

SME Climate Hub x CDP: 

https://www.cdp.net/en/articles/companies/smes-equipped-to-join-race-to-net-zero-with-dedicated-climate-disclosure-framework

SECR: https://www.carbontrust.com/news-and-events/insights/secr-explained-streamlined-energy-carbon-reporting-framework-for-uk