Mineral oil test bookmark: https://youtu.be/2DKasz4xFC0?feature=shared&t=873

It's a non destructive test. Quote from the video (with funny youtube transcription spelling errors):

"Without getting too technical, the gist is if you put the mineral oil in a vessel made of boroilic and then dip another glass made of boroilicate into it, that glass will seem to disappear while others will not. So I filled a vintage what I think is made of borosilicate Pyrex vessel with mineral oil. Then it dipped in a vintage what I think is made of boro silicut loaf pan and it seemed to disappear right before my eyes. Eureka I thought the experiment works well until I dipped a new Pyrex piece lowercase that I know is not made of borocyic and it disappeared too. Once again, another spokesman at the Cording Museum of Glass that I reached out to said that even the mineral test isn't a sure thing. According to Brady Spalling, he says in order for glass to quote unquote disappear in oil, the glass being submerged must have a similar refractory index, which allows light to pass through both without significantly bending. Mineral oil and borosyic do have similar refractory indexes. So what you've heard is correct. This method is often used to quickly ascertain whether a glass object is borosyicate. However, variability of glass recipes makes it difficult to rely solely upon this method. In short, it may work and it may not."