Form Validation Default

Hi,
How can I extend the form validation throughout all my controllers but with a twist:

I have a controller called "article", one of the items that it accepts is a "slug" I would like to do a check and make sure the slug that is used is unique.

I can do this by creating a function in the article controller however I would like to make something more universal which I can call it "slug_check" and use this in multiple controllers. The issue is that I can also extend BUT each has it's own table in the database to check, for example for "article" I need it to check in the "article" table but if we have a "gallery" that also has a slug we need the "slug_check" to check BUT on the "gallery" table.

What is the best way to do this in a fashion where I do not need to create the function each time inside the controller?