When I enlisted into the US Army in 2006, a hearing test was done for all of us as part of the initial entry stuff. Then we had hearing tests every so often since, maybe every year or two. In my case, and I suspect for most, any hearing loss while in service is likely to be documented. I did not have any, and I also didn't use the specific earplugs the lawsuits are about.
Although that does require the hearing tests to be done correctly. It was hard to tell sometimes if I heard the tone or imagined it.
It sounds like nearly 25% of people have a reference point to start with to prove that their new normal is sufficiently worse than what they started with:
"Nearly a quarter of the plaintiffs ... reported their condition in hearing tests before they ever used the Combat Arms earplugs."
That would seem to mean they have a "before" measurement, and can now test to get an "after" measurement.
Even with all that, almost everyone loses hearing as they age, so you'd really have to shnow not just loss, but loss beyond what'd be expected from a population being nearly 30 years older.
normal is a normal range, unless you can prove your "new" normal is sufficiently worse than what you started with