The Kosher designation can usually be relied on. If it doesn't say Dairy you can be pretty sure that it isn't dairy. However, mistakes do happen, see (for example) this alert from the OU (Oct 12, 2011):
Kashruth Advisory: Bean & Body - Canned Coffee Drinks
Bean & Body
Canned Coffee Drinks
Some of the cans are labeled with a regular OU, instead of an OU-D. The products are clearly dairy as listed on the ingredient panel. Corrective action is being taken.
It is always a good idea to subscribe to the Kashrut alerts from the different Kashrut organizations, to keep up with any mislabled foods. All the major kashrut organizations have them. You can find them on their respective websites.
Also, if this is the first time you're seeing the item with a Hechsher, it's always a good idea to examine the ingredients to make sure everything is copasetic. If it sounds too good to be true, it may well be.