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

I have 2 miltimeters, one with a 10A ammeter function. I hooked up an inverter and light bulb for a load and measured the DC current, IIRR it was 8A or so. Then I ran the battery down to 12.00V, checking the current and voltage every hour or so. My 4 Y.O. G29's calculated out to 90AH. In actual use the capacity would be a bit more since I can go 2 days on a full charge.