I believe you need to look at the Elements of the Document. Use the Document.getDefaultRootElement() method to get the first root Element. From there you can iterate through all the Elements. You should then be able to check the attributes of the element to see if it contains the Bold attribute using something like:
AttributeSet attributes = element.getAttributes();
if ( attributes.containsAttribute(StyleConstants.Bold, Boolean.TRUE)
// do processing here