Electronic loads are the basic tool required. They discharge a battery and record how many Ah it really has.

You'd need to make an adapter for easy access to the contacts and then connect it to an electronic load. This is a low-end hobbyist load and this is a mid-range hobbyist load. The high end hobbyist loads start around $200 and the professional units start around $500.

It is very important to set the low voltage cutoff when using a load so that the battery is not dangerously overdischarged.