You want to be careful about removing wood finish, as you can damage your wood if you use a technique that doesn’t work.
Taking off wood finish is key before you can stain or paint that wood. Though the process of removal can differ depending on the finish, many dissolve with denatured alcohol.
But if it’s the wrong finish, denatured alcohol can damage your wood.
For my purposes, the most important distinctions are between shellac, lacquer, and varnish.
First you’ll want to spot test a hidden area with the denatured alcohol.
If the finish dissolves, it’s a shellac finish.
If it only mostly or sort-of dissolves, it’s most likely a combo of lacquer and shellac. If this is the case, test it with some lacquer thinner mixed in as well. It should completely get rid of the finish.
If the denatured alcohol doesn’t affect the varnish on the wood, then apply lacquer thinner in a hidden section to see if that results in rough and then smooth surface area. If it does, the finish is lacquer.
However, if the finish roughens but doesn’t get smooth again, then it’s actually a varnish. And it’s a varnish, if neither lacquer thinner nor alcohol dissolves the finish.
Here are a few excellent guides in wood restoration if you have further questions: