If you have a fair idea of the average load you intend to power, for
example, 15 amps of 12VDC, then charge up the battery (see Steve's message
below about specific gravity or voltage for 100% charge, although I use 12.8
or 12.9 for full charge on my batteries), then use a lamp or power resistor
or constant current device that draws your desired load (15 amps in this
example). Monitor the battery voltage and note how long it takes to drop
down to 12.2 V and stop. The amps you draw times the hours to drop down to
12.2 V is the amp hours you can use at that current draw (assuming you
follow the 50% capacity usage rule of thumb).

A charged battery will self discharge over time. You can check manufacturer
specs for your battery, but it should be weeks or more if it's any good. If
it's only hours or a couple of days, I would say the batteries are pretty
much shot. Note that the longer the battery sits without use, and the lower
the voltage gets (certainly below 50% depth of discharge) whether used or
not, the greater the chance it will begin to sulfate.

http://www.batterystuff.com/tutorial_battery.html#6:

 6. Battery Testing can be done in more than one way. The most popular is
 measurement of specific gravity and battery voltage. To measure specific
 gravity buy a temperature compensating hydrometer and measure voltage,
 use a digital D.C. Voltmeter. A good digital load tester may be a good
 purchase if you need to test batteries sealed batteries.

 You must first have the battery fully charged. The surface charge must
 be removed before testing. If the battery has been sitting at least
 several hours (I prefer at least 12 hours) you may begin testing. To
 remove surface charge the battery must experience a load of 20 amps for
 3 plus minutes. Turning on the headlights (high beam) will do the trick.
 After turning off the lights you are ready to test the battery.


 State of Charge Specific Gravity Voltage (12V 6V)
 100% 1.265 12.7 6.3
 *75% 1.225 12.4 6.2
 50% 1.190 12.2 6.1
 25% 1.155 12.0 6.0
 Discharged 1.120 11.9 6.0


 *Sulfation of Batteries starts when specific gravity falls below 1.225
 or voltage measures less than 12.4 (12v Battery) or 6.2 (6 volt
 battery). Sulfation hardens the battery plates reducing and eventually
 destroying the ability of the battery to generate Volts and Amps.

 Load testing is yet another way of testing a battery. Load test removes
 amps from a battery much like starting an engine would. A load tester
 can be purchased at most auto parts stores. Some battery companies label
 their battery with the amp load for testing. This number is usually 1/2
 of the CCA rating. For instance, a 500CCA battery would load test at 250
 amps for 15 seconds. A load test can only be performed if the battery is
 near or at full charge.

 The results of your testing should be as follows:

 Hydrometer readings should not vary more than .05 differences between
 cells.

 Digital Voltmeters should read as the voltage is shown in this document.
 The sealed AGM and Gel-Cell battery voltage (full charged) will be
 slightly higher in the 12.8 to 12.9 ranges. If you have voltage readings
 in the 10.5 volts range on a charged battery, that indicates a shorted
 cell.

 If you have a maintenance free wet cell, the only ways to test are
 voltmeter and load test. Most of the maintenance free batteries have a
 built in hydrometer that tells you the condition of 1 cell of 6. You may
 get a good reading from 1 cell but have a problem with other cells in
 the battery.

 When in doubt about battery testing, call the battery manufacturer. Many
 batteries sold today have a toll free number to call for help.
 
 A novice question for you... If to test the quality of a battery - is to
 charge and discharge the battery - how many volts should the battery
 read on an ohm metter? And how long should the charge last (for a good
 battery) - hours/days/weeks/months?