What should we do to ascertain that meat is halal in America?

If a Muslim knows that the meat was slaughtered according to Islamic law, it is permissible to eat it. However, if there is doubt about the method of slaughter, one should consider the predominant practice:

If the predominant practice is that slaughter is not done according to Islamic law: It is not permissible to eat it, because the default ruling for slaughtered animals is prohibition. If the predominant practice is that slaughter is done according to Islamic law: It is permissible to eat it, because Allah has made lawful the slaughtered animals of the People of the Book.

The prevailing custom is what matters when the slaughterer is unknown. So, if the majority of the people in a town are those whose slaughter is permissible (Muslims or People of the Book), then eating the slaughtered animal is permissible. But if the majority of them are those whose slaughter is not permissible, then it is forbidden.