How to test if a sports mouth guard is durable?

By use. Be aware that all mouth guards will wear out. The biggest problem may be using them too long. Depending on the sport and duration you should get a new guard every season and probably mid season as well. If you are s bruxer or chew on it a lot plan on replacing once a month.
Dentist. Ask your Dentist to evaluate. Know that a custom mouth guard made by your Dentist will be more effective than any OTC product.