Specialized Allez Sport Help!

The only thing that changed between 15 and 16 was the paint

Your link doesn't work for me, but try googling for the model/year, then compare photos. A search for Specialized Allez Sport 2016, then Specialized Allez Sport 2015 throws up a good few hits.

As has been already suggested, the definitive method is to ask the manufacturer about the serial number. Asking for the serail number is always good anyway, as a genuine seller will provide it without issue, but more dubious sellers will try and avoid telling you.

Ask yourself, is there a BIG reason as to why it makes any difference.  Some years have different model versions but there is no real difference between them (perhaps the odd rebranding here or there) so maybe getting a 2015 instead of a 2016 doesn't really matter?  However, some years vary greatly due to major overhauls (say, jumping from SL5 to SL6 in the case of Tarmacs recently).

Perhaps it could be that the seller is telling the truth in that he bought the bike in 2016, although it may have been a 2015 model?  It can get confusing with 2019 bikes now on sale in some shops, next to bikes from 2018...and old stock from 2017.

As it happens, my winter bike is a Allez E5 from 2011 and I have put over 20,000 miles on the clock...it is still going fine, albeit that it is a bit like Trigger's broom having had the groupset replaced over time (bit by bit), different wheels (several times), new saddle, replacement seatpost...  

Get the serial number of the bike from the owner, then email Specy Support. They will tell you the year it was made.