Lacquer, especially nitrocellulose lacquer, is a 'hot' finish; each coat melts the previous coat and completely combines with it. This is the reason you can sand a nitro-finished piece and not get witness lines.
You can test the finish with lacquer thinner in a hidden spot; if it solvents the finish, it's lacquer. It won't touch the poly's.
Touching up with lacquer is pretty easy because of it's 'hot' properties; poly's are much more difficult to spot-repair.
Larry