Use the glass markings, stamps, and logos on the pieces themselves to identify when the glass was produced. A glass item’s quality is dependent upon many factors, and age is one of them. There are different methods for testing an item's age: using the markings on the piece or through testing to determine its boron content, which can then be compared with known data for similar pieces. The two primary types of pyrex are translucent and opaque. Translucent pyrex can be identified by a little T inside a diamond etched onto its base.