Definitely don't drink the water if the locals don't. Aside from this travel guideline, it's tough to judge when it is safe to drink the water in another country. It is often best to rely on bottled water, which is available almost everywhere now, and is cheaper in other countries than in the United States.
Are you safe if you just drink bottled water? Well, was the salad you ate washed in the local tap water? If so, you may not feel well later. If they don't post a sign saying the food is washed in purified water, ask them. Otherwise, it may be best to skip the salad. On the other hand, the Latin-American custom of dressing salads with lime juice may actually kill the bacteria from the wash water. I've taken my chances, with good luck so far.
Ice cubes are often the cause of water-borne illnesses among tourists. If you're not sure that the ice cubes are made with purified water, order your drink without ice. Also, brush your teeth with bottled water to be safe. Showering is usually safe unless you stand there with your mouth open.
Use common sense when choosing a restaurant. The kitchen won't be cleaner than the customer area. In Mexico I saw a box of meat sit unrefridgerated for three days. It was still for sale when I left town. It occurred to me that although I wouldn't buy meat from that store, the restaurant I ate at might. Notice signs of cleanliness, and wipe the edge of our glass before you drink.
My wife and I eat almost anywhere, and have so far been healthy during our travels. We always have a small plastic bottle of waterless hand-sanitizer gel handy, and we use it before meals when traveling. It often isn't the food that gets you sick, but the bacteria from your hands, which have been touching money and other biologically active things all day. Wash your hands a lot, use sanitizer, and you'll cut the risk of illness in half.
Not sure which restaurants are clean and safe? Ask the locals. Also, watch to see what local residents do when eating. Do they refuse the milk, or do they bring their own soup spoons? If you're in an area where the standards of cleanliness and food safety are just plain lower than you like, you can look for a restaurant owned by someone from a country with higher standards.
For total safey when you travel, don't drink the water or eat in restaurants at all, or just don't travel. Seriously though, you can eat quite well from packaged foods, and you can bring iodine pills to make purified water. Eat fruit washed in your iodine-water, and you can even maintain a healthy diet while traveling.
Steve Gillman hit the road at sixteen, and traveled the U. S. and Mexico alone at 17. Now 40, he travels with his wife Ana, whom he met in Ecuador. To read their stories, tips and travel information, visit: http://www.EverythingAboutTravel.com