On the left is a vintage white t-shirt. Note how there is a single row of stitching through the fabric. On the right is a contemporary t-shirt. Note how there are two rows of stitching.
Single stitch construction was the predominant method of t-shirt manufacture up until the mid 1990s. Since then, most t-shirts are made with a double stitch. So, single stitch construction can help determine whether a shirt is vintage.
Of course, many double-stitch t-shirts are both vintage and valuable. And many single stitch t-shirts are contemporary and common—many brands, from Ralph Lauren to Aeropostale, use an exaggerated single stitch construction to replicate a vintage look.