(1) The obverse has the portrait. If neither side (or both sides) have portraits, try to apply condition 2.
(2) The obverse side is different. In other words, the obverse side doesn't have the "common type." A good example of this is the Euro coin, which doesn't have a portrait. However, each country has its own design on one side, with a design common to all countries on the other side. The coins are said to share the same "reverse," more or less by mutual assent among collectors.
This rule would also apply to a country that has, say, a coat of arms (or some other common device) on its coinage (but no portrait.) If the common device appears on multiple denominations, the side without that device is the obverse.
(3) The side that bears the name of the country is usually considered the obverse in cases where the coin meets neither 1 or 2 above.
(4) Look at a proof set. If you have access to a special mint-issued set, such as a proof set, you can determine which side the mint considers to be the obverse because this side will be face up in the proof coin holder!
(5) Look it up in the "Standard Catalog of World Coins". I list this option last because if you had the book, you probably wouldn't be reading this checklist. Unfortunately, collectors disagree about many of the coins that don't fit one of the categories above, (and some that do), so whatever the book says about these coins should be taken with a grain of salt.
Do you have any tips regarding how to tell the obverse of a coin? Come share them with us in the forum.