The simplest, most obvious way to get information about how your employees are feeling is to just ask them. Are they getting what they want out of the employment relationship or is there some gripe about working conditions that you can correct? The most obvious time to do this is if you conduct annual or semi-annual performance reviews, in the context of discussing the employee's pay raise. As a part of that process, you can bring up the issue of what the employee likes and dislikes about the job, and the general working environment.
You must recognize that workers may not always be honest with you, either because they are afraid that you, as their boss, may retaliate, or because they don't really know why they are unhappy. But in most cases, you'll learn enough to be able to make changes where necessary.
If you suspect that morale is a serious problem among your employees and you don't know why, you can make a point of taking one of your most trusted employees aside and asking his or her opinion. Or, you can do what some larger companies do:
We suggest that you think long and hard before taking either of these two steps, as employees will often take them as confirmation of their suspicions that something is wrong, and you'll raise their expectations as to your intention and ability to make significant improvements in their jobs. If you can't or won't deliver significant changes, they may feel even worse than before.