I purchased a Dell Inspiron 15 Series 5000 (i7 4200U, 8GB ram, 1TB) from a boutique. Based on the specs it says no OS installed.
When we purchased it, the box was still sealed. However when we destroyed the seal on the box, the laptop has plastic covering that has some 'holes' in it like a tape was placed on the plastic then removed then retaped.
Then when I powered on the laptop, Ubuntu system configuration appears. I'm being paranoid thinking that it might be second hand (but it looks like brand new/no scratches in it). It's my first time to bought a laptop without 'OS' accdg to the vendor.
Apart from that, you can also check if somebody manually installed Ubuntu (then it has to be asking user id and password) or is it OEM installation (in which case, it would say something like 'it is first time you've started the computer and now is time to configure user id and password').
Once logged in, you can also check system up time and previous boot occurrences(if any).