Basically what the title said. I need to display a sub album nav list on album pages, but only if the specific album contains sub albums. I cannot figure out the correct if statement. I think there's a way of getting the number of sub albums, but I'm not sure how to use it in this case.

Thanks in advance!

Activate the print_album_menu plugin.

Then you could use something like this:
printAlbumMenuList("list-sub",NULL,"","menu-active","submenu","menu-active",$allalbums,false,false);

Also have look in print_album_menu.php ([i]zp-core/zp-extensions/print_album_menu.php[/i]). The functions are well commented.

You can even look online on our documentation:
http://www.zenphoto.org/documentation/plugins/_zp-extensions---print_album_menu.php.html#functionprintAlbumMenu

Thanks for the fast response guys. I wasn't being entirely clear in my opening post. I know how to make the list, but I also wanted to include a heading above the list (e.g. "categories, or something like that), which at this point is displayed on every album page, also those without sub albums. [b]But I changed plans a bit, so I've got a follow up question.[/b]

On my album page, I want to display all images out of both the actual album and its sub albums. The sub album menu is then used to 'filter' these images. What would be perfect, is to have a list of checkboxes that correspond to the sub albums, and to omit the pictures from a specific sub album if the checkbox is unchecked. Am I thinking about something unrealistic here? I don't know if this is only possible by doing something with dynamic albums, or if there's an 'easier' way. Cheers!

Well, "unrealistic" depends on your abilities as you would have to create such a solution yourself as we have nothing similar.