There are two things I can think of doing.
If you have a simple search phrase that you think captures the topic you're interested in, you can use a search. Let's say you want to write about fashion photography. Search for [site:hubpages.com "fashion photography"] in Google. That will find you any pages on hubpages that contain the phrase "fashion photography."
That kicks back 184 results, so I wouldn't say there are too many articles on that topic, considering there's a lot of stuff you could write about fashion photography.
If your topic exists as a sub-topic in the HubPages category system, you could also try using that. Go to "Explore -> Hubs" at the top. Find your category on the left. So let's say you went to "Arts and Designs," and then, "Photos and Photography," and finally "Fashion Photography." Then click on a hub.
At the top of the hub, you'll see some breadcrumbs listing the categories, and there is a number after each one telling you how many hubs exist in that sub-topic. There are only 22 articles in the fashion photography sub-topic, so it's a pretty empty field.