Check whether a MacBook is new or not I have bought a new MacBook Pro 13, 2015. 

How can I check whether it is a new or used one? I don't mean optical checks. Maybe some kind of system values, etc.
The only thing I can think of that cannot be tricked by an Erase and Install of the OS is the cycle count on the battery, provided it's the original battery. Essentially this number tells you how many times the battery has been depleted and recharged.

Cycle count of the battery:
Click the Apple menu and then 'About this Mac > System Report... > Hardware > Power' and look for "Cycle Count" under 'Battery Information > Health Information'

The lower the number the better. Now there is a chance that the reseller you bought it off of replaced the battery, but it's highly unlikely. It's a lot of work to remove and reinstall a battery in these models (see the full 34 steps here) so unless the battery is faulty or nearing the end of its life cycle, it just wouldn't make sense to replace it.

You could also call Apple Support and ask if the serial number of that machine has been previously registered under a name other than yours. They cannot tell you the previous name or any other information on the account if it has been registered before (due to privacy laws), but I don't see why they couldn't just let you know if the machine has already been registered with them in the past.