You don't know if one is getting reset at all. There is no way to know for sure, but if you want to get a rough estimate gamertags are eligible to get reset after 7 years of complete inactivity. The best way for you to check is by searching it on the halo 2 leaderboards. Yes there are flaws. Your best bet is to try a shit ton of gamertags hoping that one has been reset.
Nobody knows if any are getting reset or when they're getting reset. Anybody saying they have a method to know when gamertags are being reset pretty much believes anything they hear and knows nothing.