How do I find the official evergreen (replayable) scenarios of the Pathfinder Society? Society rules only permit a player to repeat 'evergreen' scenarios for credit; the vast majority cannot be replayed for credit.
Check on the Paizo Store.
One of the sections in the Paizo online store is the Replayable Scenarios section. This has all of the currently-purchasable Tier 1-2 scenarios that Paizo sells. It's not a complete list on it's own, but it is certainly authoritative. To find the others...
Ask on the Paizo forums.
I hate giving an answer like this, but after an hour of searching online I'm pretty confident in saying that there are no easily-accessible lists of Pathfinder Society adventures outside of the Paizo store. According to this thread from March 2016, there are several adventures that count as evergreen that I can't find on the Paizo store. Since the list of applicable scenarios is going to change season-to-season, in this case the quickest way to get a pretty good list of evergreen adventures would be to ask on the general Pathfinder Society forums.
As a final option, it's also possible for you to check if a particular adventure is evergreen yourself. The rule for what counts as evergreen is pretty simple:
All Tier 1 and 1-2 adventures can be replayed an unlimited number of times with a 1st-level character for credit.
So, if you have an adventure that's either Tier 1 or Tier 1-2 and is playable in the current season, then that adventure is evergreen.