Learn what properties borosilicate glass has.
Borosilicate glass is a type of heat-resistant glass that can withstand both thermal shock and high temperatures. It has a low coefficient of expansion, which means it's resistant to thermal shock. The boron in the glass bonds with the silica and makes it less reactive, meaning it doesn't break down or leach into food as easily as regular glass. This property makes borosilicate an ideal material for cooking utensils like baking dishes and measuring cups because the food won't absorb any impurities from the container itself while being heated up in the oven or on top of your stovetop burner.
Because borosilicate is resistant to chemical corrosion, you can use it to store acidic foods safely without worrying about them affecting their storage containers over time—such as lemons or limes stored in juice glasses inside your refrigerator door!
Look up the manufacturer.
To determine if the glass is borosilicate, you'll need to look up the manufacturer's website. If you're not sure who made your glassware, use Google and search for "glassware manufacturer" or "glassware brand." Once you have a list of possible manufacturers—including the brand name—look up their websites and search for their catalogs. It's likely that these are available online as well; if not, contact them directly to request one.
The next step will be looking through each catalog from 1969 to 1979 (the years in which Pyrex was sold) until you find something similar to what your piece might be. This can be difficult because many brands had similar designs during this time period (such as blue patterns), but don't stress too much if it doesn't match perfectly!
The final step is comparing prices between different models on a site like eBay or Amazon; if something seems too cheap compared with other listings, there's probably something wrong with it!
Ask the seller you bought it from if they know what kind of glass it is.
If you're concerned about the type of glass, ask the seller if they know what kind of glass it is. If they don't, it's probably not borosilicate. You can also ask for a certificate of authenticity or a certificate of analysis. Another good thing to check for is a copy of the MSDS sheet (Material Safety Data Sheet).
There are ways to tell whether or not something labeled as borosilicate actually is
The first thing you want to do is look at the physical appearance of the glassware, including its shape and color. If it's a clear-colored item, check how thick the glass appears: true borosilicate will be very thin, while cheaper alternatives tend to have thicker walls.
You can also test for thermal shock resistance by placing an ice cube on top of your sample—if it shatters or breaks apart when you remove it from under (or if it's already broken before you start), then chances are that you're dealing with something other than true borosilicate. Of course this method won't work if you're testing one of those fancy scientific beakers I mentioned earlier; but luckily there are still other ways!
If you want to know whether or not your glass has the properties of borosilicate glass, there are a few things you can do. First, look up the manufacturer and see if they have any information about their products online. You can also ask the seller if they know what kind of glass yours is made from. Finally, if all else fails, consider doing further research on Google!