So how do we tell the difference? (I'm going to talk about modern pieces for the most part, because vintage lengths tend to get more complicated.) The easiest way to tell modern pieces apart is the length. That being said, there is no hard and fast rule about length, but we will use the most common measurements. A hiraki nagoya obi is usually between 3.5m and 3.8m, whereas a share fukuro is usually 4m to 4.2m. When we start looking at vintage pieces, the lengths are typically shorter, but it is still usually proportionate to the modern pieces. For example, a vintage hiraki nagoya may be 3m long and a vintage share fukuro may be 3.5m. We also need to examine the threads used. If you see metallic thread and the length falls into nagoya style, you likely have a hiraki nagoya. Share fukuro will never have metallic threads. If you are still unsure of what you have, my best advice to ask someone more knowledgeable what they think.
Thanks for reading, and I hope this has helped you a little in your adventure with kimono!