Several ways to decide which is currently on the surface. If the table is in it's original finish, probably lacquer as polyurethanes are difficult for a manufacturer to produce quickly.

Another is if you have allowed an iced glass of something to sit and it has left a white ring. That would be lacquer finish.

Yet another, slightly more difficult is to get a small amount of lacquer thinner and rub an area. If the finish softens quickly that would be a lacquer finish.

If you were familiar with sanding each, that would be a way to tell. Problem is, the two act similar when sanded and one needs experience to know which.

Applying a Polyurethane finish over a lacquer finish is not suggested. Poly's are quite finicky and snobbish. They will not hold with any but their own kind. Some manufacturers still produce a "quick dry" oil type varnish which will act as a bonding coat but these are difficult to find.

If the surface is lacquer, check with a local paint dealer to see if he can get you in contact with someone whom will spray the table for you.