How to check originality of parts in a second hand Mac mini?
There is only four components that can be interchanged in the 2014 Mac mini:


AirPort card
PCIe SSD
5400 RPM Hard Disk Drive
CPU Fan


Other than those four items (maybe the modular power supply, but it’s proprietary to Apple) everything is physically integrated into the logic board.

Of the four items I listed, none are “genuine” Apple to begin with


AirPort card is a PCIe, Broadcom network adapter made for Apple.  Technically, you could use a different PCIe adapter, but why?  Apple always put the latest 802.1x product available at the time.  They are “too inexpensive” to swap with another.
The SSD is a Samsung with a proprietary PCIe Interface.  There are adapters out there to interface standard PCIe SSDs with Apple’s interface, but again why?  This is a Fusion Drive setup and it’s much easier to upgrade the spinning drive which....
The Hard Disk is an OEM from Seagate.  It’s very common to change this out, even for another SSD.  So, if it's not the original, there's really no harm/no foul here.
The fan is a standard fan that you would find in any off the shelf laptop, albeit, it’s one of the higher quality ones.  However, it makes no sense to to not use Apple OEM, because these are so inexpensive.  It would cost more (much, much, more) in time and labor to change out with a lessor model than the profit you'd make doing so.


Modifying System Report. 

Can you modify the System Report?  Technically yes, you can.  

For instance, by editing ~/Library/Preferences/com.apple.SystemProfiler.plist with a utility like TextWrangler, I can change the "About This Mac" model info.

But again why? Of the components I listed, none are genuine Apple and the fan doesn’t even report.  If the memory was user serviceable like previous models, you could add any compatible 3rd party memory.  The memory today, like back then isn’t made by Apple so System Report info here would be moot.  

As for the Model or even the CPU info, that info can be easily cross checked:


CPU Info can be obtained using sysctl.  See this answer for more details.  
Cross reference your model with a SN# lookup:


via Apple Check Coverage Site
via EveryMac.com



Keep in mind that the SN# is printed on the back or bottom of Mac products.  Even if "About this Mac" information were to be spoofed, it's very easy to use another avenue to verify what you have.