Check if page is filterable

Question

I'm building a store responsively and for the top navigation on a mobile I have an option to "filter" which should only be shown on pages which are filterable, such as the search results page, the catalog pages.

This is just a link with an id which makes a siderbar slide in/out. But the link only needs to be shown if there's actually something to filter.

I've tried looking in to the Catalog_Block_Layer class and there are a couple of methods which I thought might work, but don't. Such as:

Mage::getSingleton('Mage_Catalog_Block_Layer')->canShowBlock()

and

Mage::getSingleton('Mage_Catalog_Block_Layer')->canShowOptions()

Any ideas how I can achieve this?