If you don't know for certain that the water is safe, always assume the worst. Reputable brands of bottled water or soft drinks are generally fine, although in some places bottles refilled with tap water are not unknown. Only use water from containers with a serrated seal - not tops or corks. Take care with fruit juice, particularly if water may have been added. Milk should be treated with suspicion, as it is often unpasteurized.
Boiled milk is fine if it is kept hygienically and yogurt is always good.
Tea or coffee should also be OK, since the water should have been boiled. Remember to clean your teeth with purified water rather than straight from the tap. The simplest way of purifying water is to boil it thoroughly. Simple filtering will not remove all dangerous organisms, so if you cannot boil water it should be treated chemically. Iodine crystals can also be used to purify water but this is a more complicated process, as you have to first prepare a saturated iodine solution.