How to check if a applecare protection plan has registered or not?

I bought an AppleCare Plan (Box) for MacBook Pro from apple shop, but I don't remember if I have activated/registered it or not. I have sold that MacBook already and can't check its status online. What I have is the AppleCare Plan's box with the serial numbers.

How can I check if that the status of that AppleCare plan?

Thanks.

Edmond