Hi,
You would have to edit the file "cart" of the view "checkout".
The stock of the product should be available in :
<?php echo $row->product_quantity; ?>
You could add it before:
</td>
<td data-title="<?php echo JText::_('CART_PRODUCT_UNIT_PRICE'); ?>" class="hikashop_cart_product_price_value">
in order to have it after the name of the product in there.
Note that there is no point in having a "stock status" like in the wishlist since you can't have a product without stock in a cart. The product would be removed from the cart automatically were that the case. So if you added a stock status to the products in the cart display of the checkout, it would always say that there is enough stock and thus you could simply write it directly in the view without any PHP needed.