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 Airport of Entry within ForeFlight, go to the AIRPORTS page and then select INFO. In the information tab in the details section, select FEATURES. Look for the AIRPORT OF ENTRY line.
Image 1: To see if an airport is an Airport of Entry, go to AIRPORTS > INFO > FEATURES.