When it comes to jewelery I can't stress enough that it is in your best interest to buy from a reputable dealer. Always do your research before choosing a gold merchant to avoid scams and rip offs. If you have questions as to whether or not a piece of gold is fake there are some things you can look for but I would suggest taking the piece to an appraisal office to get a definite answer. Many times the answer may be quite obvious. If the ring has left a green ring around your finger, then you can bet the house it's a fake. However, this reaction may not always happen so I will give some other indications.
The first thing one should pay attention to is the stamp inside the ring. If your ring has no manufacturer engravings, you may be looking at a fake. It is required by law for any metal to be stamped with a number for quality. If you look inside the band of the ring you should see a stamp with something like 16k which would indicate that the ring is 16 karat gold. For older rings and estate jewelery a missing stamp wouldn't necessarily indicate a fake so you may want to double check with a jeweler.
If you have a white gold ring there are a few things you should know. First of all, there is no such thing as "white" gold. Gold is always yellow. To achieve the look of white gold, regular yellow gold is mixed with other alloys to achieve a silver color. After that the gold is plated in rhodium to give it a final silver or platinum looking luster. As the rhodium wears away, the ring will appear to yellow. This may concern some people. The yellowing of the ring does not indicate that the ring is fake by any means. It simply means that the actual real gold is beginning to show through due to the rhodium deteriorating. All you need to do is take the ring in for a cleaning and a new coat of rhodium and it will look as good as new.
One way to test gold for authenticity is a bleach test. The chlorine in the bleach will react with real gold causing a color change. When real gold is placed in bleach it will change to a pale gray color. The problem with this test is that though it will show you whether or not your ring is real, it will ruin it in the process. Though yellow gold is not too dangerously affected by the chlorine, your ring will need a really good buffing by a professional to fix the damage done by the chlorine bleach.
Since you will have to bring the ring in for buffing anyway, why not just skip the bleach and let a professional apraise the ring for you. Some people believe that since gold is softer than other metals you can test the ring by biting it. The theory is that if you can see your teeth marks then the ring is most likely real gold. I wouldn't necessarily trust this test to a novice jewelery owner. I would suggest that you bring your ring to a professional for appraisal if you do not see any stamp on the inside band. If your ring does have a stamp but you are still suspicious, it wouldn't hurt to have it checked out anyway.