Although use of ultrasound to locate the epidural space is increasing very few people do real time ultrasonographic guided epidural block. 
Mostly it is used to assist epidural or spinal block.

The uses are:
1. To locate the midline. In patients with high BMI it is sometimes very difficult to feel the midline. 
Ultrasound can help in finding the midline.

2. To locate the correct interspace to do the block. The level of intervertebral space can be easily determined by counting upwards from the sacrum.

3. To measure the depth of epidural space and dura from the skin.

When we scan to locate the epidural space, essentially we are scanning over bones (vertebrae) to find a gap inbetween the bones (intervertebral space).  Through this gap we try to see the ligamentum flavum, dura and the epidural space. 

Before we see how we can use ultrasound to see the epidural space, let us first see how a bone looks under ultrasound. 
Bone does not allow ultrasound to pass through it. 
Therefore all the ultrasound that is incident on the bone gets reflected by the bone. The periosteum of the bone which reflects the ultrasound back to the probe produces a bright hyperechoic line. 
Since no ultrasound travels beyond the periosteum, the rest of the bone produces a dark shadow. 
If a structure is behind the bone, ultrasound will not be able to reach it and thus that structure will not produce an image.

The image of the lumbar vertebra is from Gray's anatomy taken from 'Wikimedia commons'  The image shows parts of the lumbar vertebra.

Let us see how lumbar vertebra will look under ultrasound:

Here we have immersed a plastic spine in water bath and scanned it using a curvilinear ultrasound probe.  This gives an idea of how a spine will look under ultrasound.

Following video is from RA-UK explaining how to look for epidural space using ultrasound:

In the following video Dr Balaji has explained how he uses ultrasound to locate epidural space.