You don't you just use it everyone has different settings, apps, brightness, wifi and Bluetooth usage etc and will get a different battery life, unless it seems to have an issue just use it and enjoy.
Your new machine may also take a couple of weeks to settle down battery wise this has been observed over and over again on here and in other sites, don't even think about it for the first week or so at least.
I don't. I just see how long it lasts for me doing normal stuff. There are myriads of ways of doing batter testing and just as many factors that will affect it. Professional reviewers have their own methods, which can be more or less realistic, but even they fail more often then succeed (like the latest consumer reports debacle...)