I just flew from Dublin airport to JFK, and pre-cleared customs in Dublin. However, I was never given the opportunity to declare anything, and I never filled out an entry card.
If I had, for instance, been carrying more than $10,000 in cash, when should I have declared it?
You should have talked to at least one CBP officer when going through inspection. Then.
CBP is the US Customs and Border Protection agency - these folks are responsible for both the passport and immigration checks, and the customs checks, all at once (in many other countries these are separate agencies and separate functions).
Trip report: Yesterday I went through pre-clearance in Toronto (YYZ) with Global Entry. The person checking passport specifically asked: "do you have anything to declare: food, vegetables, fruits?". There was no other customs interaction either in Toronto or in the US.
So I guess the answer is "You declare it to the person checking your passport". They are probably supposed to specifically ask you, but they don't always do.
Caveat: this process seems to change regularly and may not be consistently applied everywhere. If you are actually planning to declare something its probably best to be proactive about it and declare to any US officer you interact with. If it's the wrong one to declare to, they will simply tell you so and provide guidance of what to do next.
