Read labels on
everything you might want to eat, looking for milk, butter, dried milk, yogurt, cheese, casein, whey, sodium caseinate, lactose (and other things that start with «lact», although sodium lactylate is non-dairy)... Read all labels — check the bread, check the hot dogs and bologna, check things that say «non-dairy» like non-dairy creamer and Cool Whip (as
mentioned above, these both
contain dairy).