Getting the posts current category ID

Ok ive tried lots and lots of things, searched extensively here and tried more sql than I care to think of but this is beyond me.

I have posts that are in multiple child categories but i want to display some random posts from the currently active posts child category. So all i need is to retrieve that posts currently active child category ID.

I’ve tried the global $cat but that just returns the parent category and not the currently viewed child category.

Is there anyway to retrieve the current viewed posts child category id ?