Reply to Pro 2 serial check

Hi - how do I double check if a serial for Por 2 is vaild/not taken ? I already bought my Pro 2 before I got my S4, so never actually used the version that came with the S4.

I actually ended up sending mine back for fixing a few months back (along with that disk I never used), and just got a brand new replacement with it.

I'm wanting to sell it on (moved on to Pioneer gear now and keeping my original Pro2 to use with my old numark as back up), but want to double check the included disk just to make 100% before I advertise it that it's 100% not used and is that serial is available.

So hoping someone may know but, what's the best way to check the serial is authentic without setting everything up and loading the license with the included S4 ?