I'm a korean student living in korea with no US address or US account so I've got no realistic way to receive my check. I haven't been on KDP for a while but I just realized I should've received a check in either 2014 or 2015. Is my check still valid, and if so how long for?
If you are paid by check, you would have to wait until you had accumulated $100 in royalties from the US store, or 100 pounds in royalties from the UK store, etc. Have you sold that many books?
You can use the Contact link at lower right to see if Amazon has ever tried to send you a check. You can find out how much you've earned by using the "Historical" tab under Reports. Sometimes Amazon will send you a check even if you haven't quite reached $100.
Perhaps you would be better off using Payoneer. I've never used it, but many seem to like it. You can find out more from threads in the Payments forum. Good luck!