Is there a real-world test of a battery's true capacity?

My two group 31 batteries are rated at 110 amp hours -- and I never discharge them past 12.1 volts, or 50 percent state of charge. So that means, I think, that I should be able to get 55 amp hours out of each one. But I would like to know for sure, and I would like to test this under real-world conditions, rather than deriving a theoretical answer or one that depends on extrapolation.

There must be a way of hooking up a device with a known wattage and determining how long it takes that device to drain the battery to a specific state of charge. I don't want to completely discharge the battery -- that could damage it, or so I have been told.

So, for example, I could hook up my inverter and plug in an old fashioned analog clock (the kind with hands -- remember?) that runs on electric power (I happen to have one) and then checking it every twelve hours, which is pretty easy to do. When I find it has stopped, I can tell exactly when it ran out of juice. But that would drain the battery.

Is there a better way of doing this? Thanks in advance for your suggestions. Really clever answers that don't require the purchase of expensive testing equipment are particularly appreciated! ;)