To make things easier for our readers, the following official links will determine your 3DS Ambassador status, categorized by region. All you need to do is enter your serial number.
1) North America, Canada, and the Middle-East: http://www.nintendo.com/consumer/systems/3ds/en_na/3dsambassador.jsp?menu=how&submenu=ctr-ht-amb-lookup
Note that while most of the North American-based 3DS serials start with a “CW”, some may begin with different letters. Middle-Eastern 3DS systems start with a “CS”, yet they play North American games so you may check your status through NA’s link as well. If your serial doesn’t work even though you’re sure that you’re eligible, try not including the final number (which is surrounded by a box).
2) Latin America: click here
4) UK: click here (not active as of 8/20)
The link is not active yet. However, UK owners who didn’t connect to the e-Shop in time will have an alternate method of being an Ambassador through this link, too. You need one of two things: a 3DS friend code through online play, or a proof of purchase receipt.