Nintendo has launched a web-based tool that lets 3DS owners check if they're eligible for the Ambassador program.
Nintendo is giving 20 free games to 3DS owners who bought their consoles before the price drop on August 12. To qualify, you would need to have logged into the eShop online store via your 3DS at least once before that date.
You did that, right? Good, so you should qualify. But if you want to make absolutely sure, this Eligibility tool will tell you - you just enter your serial number on the back of your console into the box and it'll tell you.
Note: Even though it asks that you include the boxed digit at the end of your serial number, that doesn't appear to work as several forum users have complained. If you have issues try leaving out the boxed number.
The tool also only appears to work for North American 3DS units. Nintendo Europe is yet to launch a similar tool.