White gold is an alloy of gold and other metals, such as silver or palladium, while platinum is an almost entirely pure white metal. In a natural state, platinum has a much whiter color than white gold. However, most white gold is plated with rhodium, which makes the colors much more similar.
The easiest way to tell platinum jewelry from white gold jewelry is to test it by weight. Platinum is much heavier than white gold, so a 14kt white gold ring is sure to weigh less than a 14kt platinum ring. Platinum also tends to be much more expensive due to its purity, which is usually around 95%.