Don't go forgetting that the Wonder Card Wi-Fi event for the Oak's Letter was distributed in Japan during May, so any Japanese Shaymin is authentic and any English Shaymin that comes from a game that gained the Wonder Card from a Japanese game is also authentic. I don't know if you can inter-language transfer Wonder Cards with Oak's Letter and such, but with the Secret Key, it was very possible because I did it to see if it would work. (I couldn't be bothered waiting for the US distribution date of the Key... -w-;; )
The only way to check the authenticity of a Shaymin any more (considering if the Wonder Card transfer works for the more recent Japanese Wonder Card Wi-Fi Events) is to make sure that the date it was captured on, at the Flower Paradise, is on or after the first day of distribution of the Wonder Card Event for Oak's Letter.