ForeFlight does not show a comprehensive list of Airports of Entry. You can view current US Airports of Entry using the US Customs and Border Protection website:
To see if a particular airport is an Airports of Entry within ForeFlight, go to the AIRPORTS page and then select MORE and tap on FEATURES. There is a line listed as AIRPORTS OF ENTRY that will note whether the airport is an Airport of Entry.
How to see if an airport is an Airport of Entry