Get current entry category I haven't used categories before so maybe I'm looking at this the wrong way.

I currently have a category group named products with the following structure:

Featured
Fruit
----Apple
----Banana
Veg
----Turnip


I know I can do the following to show just the subcategories of fruit:

for cat in craft.categories.group('productCategories').descendantOf("fruit ID")


But how do I get the "fruit ID" when I'm on the on the fruit page?

 craft.categories.relatedTo(entry)


returns all the categories in products but I want just the current category.

Any help greatly appreciated!
I take it you are on the category template as defined in admin/settings/categories/:id.

You should have a model representation of the category assigned to category in the template, so you should be able to get the current category id by simply doing {{ category.id }} in the template.

Your code could look like:

for cat in craft.categories.group('productCategories').descendantOf(category.id)