There are a few cyclists that prefer to work on their own bikes while others would rather leave it to the professionals at their LBS. If you have the tools to perform your own hub service and are willing to give it a try we did a post on a cup and cone style hub last week that you can view here.
Here are a few signs to look out for that indicate that you should give your hubs a little TLC.
Poor rolling capability
Any play on a securely fastened wheel
To accurately inspect your wheel start by removing it from your bike. Have somebody assist you in spinning each wheel while holding the axle firmly in your fingers. This removes any additional bike noise and will allow you to feel and hear any excessive resistance on the bearings and freewheel. After you have cleaned and re-greased your hubs try this process again. If you are still experiencing problems rather be safe and take it in to your LBS to ensure you always have a safe riding experience.