There is one absolute about replacing batteries. If one battery is bad, replace them both. Batteries have to be matched, and if one battery is old and the other one is new, the batteries are not matched.
There are two absolutes about testing batteries.
If the batteries are warmer than the panel that they're in, the batteries are bad and have to be replaced. A cool battery does not mean a good battery, but a warm battery is is one that is constantly needing a charge, or it is being over charged. Replace both batteries if one is warm.
Look at the date that the batteries were installed. If the date shows that the batteries have been in service 4 years or more, just replace the batteries. The NFPA says that batteries last an average of 5 years from the date of manufacture, so they should be replaced by the 5-year anniversary from when they were manufactured, not from when they were installed.
You can assume that the batteries sat on shelves in the store and in the shop for months, so, by the time you're back to test them in a year, the 4 year old batteries will be past the time the NFPA says to replace them. If they've been in the fire alarm system for 4 years, automatically replace both batteries.
The idea about testing batteries is to find out if during a blackout they will provide power to the fire alarm system for at least 24 hours, and also, at the end of 24 hours, provide the extra power needed to sound the alarm.
Checking current from the batteries does show how much current the panel is going to use from the batteries. The problem is that measuring current doesn't show how much power the batteries will be able provide for the panel.
There is no absolute way of testing a battery, but the best way (if you actually had the time to do this) is to turn off the utility power to the fire alarm system for 24 hours, and if the system has horns and strobes, sound the horns and strobes for 5 minutes. Then, if the batteries are around 22 volts or higher, the batteries are good.
The next best way is to use your judgment. What you do is measure the voltage on the batteries before testing. Because that is the charging voltage for the panel, the voltage will be a little different from panel to panel. While utility power is still connected, measure and write down the voltage.
Then turn off the utility power. Measure and write down the voltage again.
Then sound the horns and strobes. Measure how fast the voltage goes down. Write down the battery voltage after a minute. The voltage will have gone down, but in about a minute the voltage should not be below 25 volts.
When the test is complete, turn off the horns and strobes and turn on the power again.
The Voltage Indicates What's Left
Some technicians use a low resistance, high wattage resistor to put a heavy load on the batteries for half a minute to a minute. They measure the voltage on the batteries before the test, and then during the test, they watch how fast the voltage goes down. Using that method is about as good as using the method outlined above. The bottom line is that it's the voltage on the batteries that shows the life that's left in the battery.
When you get down to it, the voltage that's left on the battery after testing is the only measurement that can be used to even get an idea of whether the batteries are good or not. Even that, though, is not a true measurement.
You will have to use your judgement; outside of the 24-hour test, there really is no proper test. After testing quite a few batteries, you will get an idea of a good battery versus a bad battery.