The bottle is made of glass but in a photo could look like either glass or plastic. How do I make it look definitely like glass so there is no question about it?
To make things harder, the glass is painted on the inside.
I'd recommend reading the book Light: Science and Magic by Hunter, Fuqua, and Biver. It has a whole chapter dedicated to lighting glass.
From memory, it says that with glass it's lighting the edges correctly that is important in coveying the qualities of glass.
(No connection to the book by the way, other than being a satisfied reader).
Use multiple diffuse light sources. 2-3 of them should be enough.
If it's not completely opaque due to the paint you mentioned, you could also try getting a shot of light coming through the glass. Perhaps a candle or such.
This is thus just a brain interpretation that differ
and use very diffuse light source
the worst (because not in picture itself) but efficient, after shoot modification on picture