To give another deviant a direct link to this FAQ answer, use the following code in your comment or forum post:
Can I browse a user's gallery by a specific Category?
Yes you can. Simply browse to the user's gallery of choice and select a Category by clicking on the 'Categories' button, located above their deviation thumbnails and the user specific search field. Next select the Category of your choice, if the Category has items in it you will then be presented with results.