Polyurethane or Varnish - How can I tell?

[Hardalee53]

It's time to refinish my teak toerail and handholds but not sure what the previous owner used. I was sure it was varnish but now I'm not so sure. I've read that polyurethane will tend to flake off. I see signs of that but only where water has penetrated underneath in a few areas where the teak is discolored or where some of the coating was slightly overbrushed on SS deck fittings. Is there way to determine for certain?

[GordMay]

Rub the edge of a coin back and forth over a hidden/unobtrusive area, while pressing firmly.
If the finish flakes, the coating is Varnish, Wax or Shellac.
If the finish is resistant to the coin, or crumbles, the coating is Polyurethane or lacquer.
Acetone will slightly dissolve Shellac or Varnish (becomes tacky), will fully dissolve Lacquer, but bead up on a Polyurethane finish.
Denatured alcohol (ethanol) will react slowly on Varnish, but Shellac will dissolve right away.