Shellac and lacquer are both fairly easy to identify because they will redissolve in their original solvents. If alcohol (solvent alcohol from the hardware store, not rubbing alcohol) dissolves it then it's shellac. If lacquer thinner softens it, then it's lacquer. (lacquer thinner will also dissolve shellac, but alcohol will not immediately soften lacquer, so do the alcohol check first.) If neither alcohol nor lacquer thinner immediately affect it, then it's probably varnish.