How can I tell what module to edit?

At cma.mtsidevspace.com I have a menu. If I click "Apparel for Men" I get all the categories under that. If I click "Graphic Tees" I get a list of the shirts in that category. I'd like to set the style to img_pane for that product listing. How can I tell what module to mess with to do that?

If you look at the HTML of that page with firebug, where you have the products, you will see the class name:
hikashop_category_information_module_73
for a div element around your products listing. The number in there is the id of the module displaying the products listing.

So you can go in the menu Display->Content modules and edit the module with the id 73 and you will be able to change the item box layout to that layout there.