So, what does someone keeping Orthodox kosher restrictions look for in the supermarket? You may not know, but there are kosher symbols printed on thousands of packaged food items. Go ahead and take a peek at your pantry, and you'll recognize some of the marks I'm about to discuss.
Since all processed products need supervision to ensure that no hidden ingredients were added to an ostensibly kosher item, a system of symbols denoting an agency dedicated to checking the processing facility developed. Common symbols from agencies in the US that are pretty universally accepted include the following:
That would be the OU from the Orthodox Union, OK from the Organized Kashrus Laboratories, the Star-K, and the Kof-K. There are dozens of smaller organizations throughout the country, as well as international organizations. If I'm in the store and see a symbol that I don't recognize, I use my handy-dandy Chicago Rabbinical Council iphone app to look it up.
Now, not all agencies are universally accepted. That would be too simple! Agencies vary in their requirements- how often does a mashgiach, or an agent of the agency, come to check on the facility? Are his visits a surprise? What other food is processed in the plant, and what is the cleaning process between runs? All of this impacts on whether a kosher-keeper elects to eat food certified by a particular agency. So if you're buying food for a friend to eat, simply ask them if they find a given symbol acceptable. There is no worse feeling than when someone has gone out of his/her way to buy something kosher for you to eat but you don't actually accept the product's certification.
The symbols also indicate the food's denomination. Foods containing even trace amounts of milk will have a D for dairy next to the symbol, and foods containing meat will indicate so as well. If the symbol appears without a letter next to it, that means it is pareve, contains neither meat nor milk. If it has a P, that doesn't mean it is pareve, but actually indicates that it is kosher for Passover. If you see a D-E, it means the product is pareve but was made on dairy equipment, so it may not be eaten with meat.
Remember, uncut fruit and vegetables are always kosher. That said, many kosher-keepers check produce in particular ways to ensure that there are no bugs within. Apples, oranges, pears and plums are safe bets if you'd like to have something on hand for your kosher friends. Keeping some disposable cutlery and plates around isn't a bad idea, either.