Un register Ipad

[petegunn]

Hi i have sky+ extra and my family between us has 4 Ipads (we don't live all in one house) . I wish to buy another Ipad. I want to de-register one of them . How can i tell whom has got which one. I have looked on sky settings,but it don't say who's got which one. is there a simple way.
Pete G

 

[J. A.]

Were they all purchased at the same day?

I can differentiate between my mini and my Air because of the date when the contract started/will end.