It all depends on what Apple store has in stock. It could be from 6.0.1 to 6.1.3.
I bought a Verizon iPad mini not too long ago, the first one had 6.1.2 but I went to exchange it because it came scratched out of the box. The exchanged one came with 6.0.1, and surprisingly I checked online and mine was built in December when I got it a week ago, but I upgraded it to 6.1.3 (I like to have all my device up-to-date) because I'm not a fan of Jailbreak.
On the box, it does have a serial number listed and you can visit this site
http://www.chipmunk.nl/klantenservice/applemodel.html and type in the serial number. It should then tell you when it was built. I suppose you could do this at the apple store but I'd say risky because salesperson may look at you weird.