Equivalent to in_category() for subcategories

try this:

function is_subcategory($category = null) {
    if (is_category()) {
        if (null != $category){
            $cat = get_category($category);
        }else{
            $cat = get_category(get_query_var('cat'),false);
        }
        if ($cat->parent == 0 ){
            return false;
        }else{
            return true;
        }
    }
    return false;
}

Usage:

//to check if the current context is a subcagetory:

if (is_subcategory()){
    //its a subcategory;
}else{
    //nope is aparent category
}

//to check if a specific category is a subcategory pass its ID, eg:
if (is_subcategory(12)){
    //yes category with the id of 12 is a subcategory;
}else{
    //nope is aparent category
}