Due to the existence of different businesses each company or firm has established it own vision or goals with regard to environmental performance. Therefore, in my humble opinion the best way is to look at the environmental objectives that each company set and then to audit their performance against these set goals. Of course there should be a roadmap or a plan to achieve these goals. Then it is easy for you to check whether the company has achieved these goals or not. I would suggest that you can look at the EMS of the company you want to assess its performance.