That's quite an ordeal. I think someone (moneromooo?) was working on a way of getting the key image from a transaction so you can check the spent status using Moneroblocks API... (like Luigi's coin checker)
In the cold wallet:
export_key_images somefilename
Copy somefilename to the hot wallet where your view only wallet is
In the view wallet:
import_key_images somefilename
As the inputs owned by the wallet are spent, the view wallet should see those spends.
Obviously, if the cold wallet receives new inputs (including change), those key images will need importing too for them to be trackable.