There are several ways to determine if glass is borosilicate: 1. Look at the physical appearance of the glassware, including its shape and color. True borosilicate glass will be very thin, while cheaper alternatives tend to have thicker walls. 2. Check the refractive index of the glass. Borosilicate glass has a refractive index of 1.474, which can be tested by immersing the glass in a container of a liquid of similar refractive index, such as mineral oil or dry. 3. Measure the density of the glass using a water bath. 4. Look at a thick layer of the glass (e.g. along the sides). Soda-lime glass has a green-blue tint, while borosilicate glass is more yellow. 5. Borosilicate glass is made by mixing 70-80% silica sand with boron trioxide. When heated, boron trioxide absorbs massive amounts of heat before crystallizing into a glass. However, just looking at it, you might not be able to tell the difference between borosilicate glass and regular glass.