Look for paint sanding scratches and "orange peel" roughness to the finish. Those are signs that finishing wasn't meticulous enough, and the new appearance may not last very long.
If the finish appears thick - as polyurethane sometimes does - it may dampen the instrument's projection and sustain.
Ideally, the instrument will have multiple coats of nitro-cellulose lacquer which were very carefully buffed by hand on a buffing wheel to take out all the "orange peel" rippling. Nitro-cellulose lacquer gives a deeper, warmer, mirror-like luster for a finish that makes a quality statement.
Also, nitro-cellulose lacquer is more lasting than water-basing lacquers available today. It is preferable to polyurethane which is commonly used because lacquer allows repairs to the finish without refinishing. Refinishing, of course, lowers the vintage value of instruments.