system
Given the following post, respond as if you're commenting on Reddit:
human
[Avatar: The Last Airbender] How does each nation identify the Avatar?

We know that Air Nomads use the toys of old Avatars, and from Rise of Kyoshi we know that the Earth Kingdom uses a geomancy technique where they narrow the earth kingdom down by a half each time. 

What are your best guesses for the Water Tribe and Fire Nation techniques?
gpt
In the Rise of Kyoshi it explains the Fire Nation method, sort of. All children are tested for bending ability near birth. To avoid firebender children from surprising their parents and burning houses down. Some how they are able to detect the Avatar amongst them. The Fire Sages then notify the Avatar on their 16th Birthday, like we saw with Roku, having known all the while.