How Can You Tell if an AGM is Really "Deep Cycle"?

I am anxiously awaiting to see how my 3 year old Lifeine 4D has survived 8 months storage, unconnected to anything & with NO maintainer during that time.

Previous periods of dormancy have usually been a little short of 6 months of no attention with no ill effect. My first Lifeline 4D had a service life of 9 years averaging 150 use nights a year.