Usually in the Apple Store online (US) the system version is noted
in product information details. The Apple Stores tend to sell new
inventories and the most recent systems; unless they have some
old stock new inventory. Independent authorized resellers may have
old stock, some at discount; well after the latest OS X is shipped.
{Old stock, in online Store, would be in Clearance/Special deals.}
Several different hardware specifications of models in production
usually can tell you what OS X they shipped with & version number.
{Each version in El Capitan that shipped with or was included with
a new configuration, is stated in the nomenclature of each build.}
macOS Sierra has been stated to be available after 20th Sept 2016.
This would most likely mean it would be available for download;
not necessarily available in stock, in retail boxed Mac products.
{But Apple likely leaves little to chance, in premier Store locations.}
So hardware would likely still be shipping with the OS X that was
available at the time of manufacture; you would have to check to be
sure that a 'new' computer model you want, was built after the date
of release -- or may have been intentionally available by that date.
Depending on your location, region or country, availability of newest
macOS relies on timely arrival of latest versions of inventory, shipped.
The good thing about getting an all-new configuration, is the Apps &
macOS, should work adequately in most recent manufactured Macs.
As this is a user-to-user discussion community, we'd be among the
last to know (or be forbade to tell, if we did know) any inside secrets.
El Capitan has been shipping for some time now, within new inventories.
In longer lived product histories, such as MacBook Pro 13-inch non-Retina
June 2012 build series, they started out in 2012 with Lion 10.7.x, & the
latest ones (two or three latest systems) were steps within El Capitan 10.11.
And that build series is still in production, the last with optical drive inside.
In any event...
Good luck in your choices! 🙂
