How can we be so sure? Besides the decades of great reviews from fellow musicians? We can tell by the weight of the case. Fiberglass is not the lightest material. In fact, Bobelock fiberglass cases weigh about 15 lbs.
There are many brands with fiberglass cello cases under 10 lbs. However, to achieve that lightness, sacrifices have to be made. Unfortunately, it's the overall protection of the case that is being sacrificed (probably the last thing you want to give up when it comes to buying a case).
A real fiberglass cello case will weigh about 15 lbs.
The Bobelock cases also come with great suspension systems, so your instrument won't knock against the case if you accidentally drop it. The suspension system is designed to absorb the impact.