Hey, bc. You have the first part of the test. If alcohol removes the finish, it is probably shellac and, yes, denatured is best.
Step 2 is the same thing with lacquer thinner. If a couple of drops really softens the finish in a couple of seconds, it probably is lacquer.
Step 3 is the age. If the finish is from before 1990, it probably is not water based. And if it looks cloudy/milky in certain light, it probably is water based.
Step 4 is to try a couple of drops of toluene or xylene. If the finish softens quickly, then it's water based and not lacquer.
If none of the above work, then it's a factory applied conversion finish. There are several different kinds but you will have a hard time removing it.
There is a lot of good info in a book called "Understanding Wood Finishing" by Bob Flexner. It is easy to read and yet the bible for amateur finishers and refinishers. It's easily available on the web or from the big booksellers but let me know if you need more info. Mine was $16.95 and well spent.