original painting oil or acrylic?

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There are a couple tests I use...

~ Painted pre-1960... it is undoubtedly an oil and not acrylic.

~ If you smell the painting and it smells of turp and linseed oil... it's an oil.

~ If you hold the canvas up to the light so that the light rakes across it and you see uneven patches of glossiness and matte, then it is usually an unvarnished oil.

~ If the painting was painted within the past week and it is dry... it is an acrylic. :D
 
I do the smell test.

However, 2 weeks ago I got an oil painting on canvas, the artist bringing it in so she should know, and I couldn't smell anything. And no, it was not a water based oil (yes, that does exist although it sounds stupid)

As for uneven patches matte/gloss; that happens in acrylics too. Not every color has the same gloss.
 
Use a high power magnifying glass or jewelers loop. In acrylics you can see tiny pin like holes in the peaks of the paint. They occur as the paint dries and air escapes.
 
And just to add a phrase to your "professional vocabulary" those pinholes are called evaporation cones. As in: "The reason you should glaze your painting is that it is covered with evaporation cones (as you can see through this here handy dandy magnifying glass that I happen to have ) which will trap soil so thoroughly that it can not be easily removed. May I suggest Optium acrylic or Museum glass?"
 
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