Originally Posted by Majuki

If you have dutiable items, declarable cash, or agricultural items, it would be incumbent upon you to seek out the correct place to address those items in the customs hall. (You can ask how to do this at passport control as the implementation can vary based on location.) If attempting to leave the customs hall with those items and stopped, it's unlikely that you'd have the ability to declare those items at that point. However, if you're standing around the baggage carousel and USCBP starts randomly approaching people and asking questions, there is the ability to proactively declare. For instance, "I am coming back from France with a couple of bottles of wine. What is the process for duty assessment?"