Although it is all in the toolset, it can be tedious. For example, with weapons, you have to look at all the colour variations, because, while many are standard or null, in some haks they are entirely new models.
For cloaks, you can just look at cloakmodel.2da in the top hak.
One method for other items is to use nwhak to select the entire range of weapon / shield models from each hak, exporting into one common temporary folder. Thanks to the naming convention, that's not difficult. Now you have all the models in one place, and can determine by inspection which model numbers are in use.
On my new project, where I have the luxury of starting from scratch, I put all new weapons into my own hak, in order to maintain a tight audit on what's in use (but that's probably too much work for a legacy module with umpteen weapon haks).