How do I tell if a forge has been equipped with a nail mould?
Assuming you have rights to use the forge in question, when you click on it to use it, you
may see a menu item saying "Upgrade...". If one of the options is "Nail Mould", then that forge
does not yet have that upgrade.
This can apply to other buildings as well; Carpentry Shops are another example of a building that
may have an upgrade.