Standby eligibility depend to a large extent on the booking class of your ticket. However, the best thing to do, as Brendan says, is go to the the connecting baggage counter as you leave Customs and ask if you can go on an earlier flight. Sometimes the agents will do this with no questions asked, or you may have to pay a standby fee that could be from $50 to $150, depending on the ticket. Regardless, if there's space you should be able to get on an earlier flight.