Apple - Check whether a MacBook is new or not

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.

Take a look at the hard drive. The SMART statu should have lots on informations.

Here is how to look at it:

Is there a utility for mac that shows the information of an SSD like the health etc…?

A samble output from my computer:

0x01  0x008  4           17080  ---  Lifetime Power-On Resets
0x01  0x010  4           13034  ---  Power-on Hours
0x01  0x018  6     71009610622  ---  Logical Sectors Written
0x01  0x020  6       689785468  ---  Number of Write Commands
0x01  0x028  6     96669153261  ---  Logical Sectors Read
0x01  0x030  6      1474232041  ---  Number of Read Commands

It's even less likely they would have replaced the hard drive than the battery :)