How many amp-hours of capacity does your battery really have?
Here's how to test the capacity of a 12 volt battery with an inverter, a lightbulb, and an electric clock. This can be pretty important to know. Will your battery last long enough to show a feature film at your guerrilla drive-in theater? Will your marker light stay on all night on your boat? I first saw this trick in the magazine Mother Earth News

WARNING
That "deep cycle" sticker on this battery doesn't mean anything. Internally, the plates just aren't the right shape to get long life from deep cycles and still put out enough current to start a car. To make a real 12 volt deep cycle battery, take two 6 volt T105 golf cart batteries and tie them together.

MORE WARNINGS
Please comment with more warnings. It's late and other people are really good at battery warnings.

Step 1: Battery Tester

You've probably seen your mechanic use this type of battery tester. It's got a voltmeter for checking battery open circuit voltage. It also has a switch that turns on a very low-resistance dummy load. You use that to test how much short-circuit current the battery can put out.

A meter like this is really handy if you're going to mess around with 6 and 12 volt batteries much.
You can buy one from harborfreight etc. with the money you get by scrapping a dead battery or two.

You can do both these functions with a handheld voltmeter and the dummy load of your choice.
A length of haywire would do nicely. But the store-bought meter probably ends up being cheaper than the haywire substitute. For one thing it's got a cage around the dummy load, so you don't get burned when it glows red.

Step 2: Voltage Test

You can't tell anything for sure about a battery until it's charged.
If it's not fully charged hook it to a battery charger until it is charged. I usually put them on a 2 amp trickle charger for a day or two. A 200 amp-hour golf cart battery might need a few days to fully charge that way.

A fully charged "12 volt" lead-acid battery is about 12.6 volts. (w'pedia)
While charging you need to drive it at about 13.5 to 14 volts to make the current flow in. The battery will float high for a bit after charging, so voltage isn't great as an indicator of state-of-charge. That's why people check the cell electolyte density with a hydrometer. That IS a good indicator of state-of-charge.

Step 3: "cold Cranking" Current Test

Push the button on your battery tester. It shorts the battery out through a low ohm resistor.
This battery is putting out 400 amps. No wonder batteries make good stick and wirefeed welders.

This is the important test for a car's starting battery. It's made to deliver high currents but not for very long.

This is a good test to use while scavenging for batteries. If a battery doesn't pass, it might just need charging. But if it does pass it's probably a fine battery.
Some batteries pass but have low capacity. That's why we need the capacity test.

Step 4: Set Up the Capacity Test Rig

Most inverters have an automatic low-voltage disconnect feature.
It makes a beeping sound and stops putting out 110AC when the battery voltage drops below 11 volts or so. You've encountered this feature if you've played with inverters much.

The plug-in electric clock tells us how long it takes to discharge the battery that far.
The hands stop moving when the inverter turns off. We'll start the clock at 12:00:00, some time later when the light turns off, look at the time on the clock to see how long it took. An old 24hour electric clock with numbered flaps for a display would be perfect for this. Or an hour-meter from a stationary engine. Or a timeclock from a factory.

This 60 watt lightbulb makes the inverter draw about 5 amps from the battery.
Watts = Volts * Amps.
60 watts = 12 volts * 5 Amps.
5 Amps = 60Watts/ 12Volts

If you want to be totally sure, check the current with an ammeter.

Step 5: Size the Load to the Battery

4 hours have gone by, the battery has put out 20 amp-hours and is going strong. This battery should be good for 80AH or so. It will take 16 hours to discharge at this rate.
If I'd used two of those lightbulbs the test would go twice as fast.
I'll have to look at the clock again in just under 12 hours to avoid uncertainty about what the reading means.

Look up how much current your battery was made to deliver. Don't overload it.
There's a relationship between cycle depth, rate of discharge, and lifetime of the battery.
Battery manufacturers can provide amazing amounts of data on this. It can be pretty confusing until you wrap your head around it.

Let me summarize it for you.
Flooded lead-acid 6 volt golfcart batteries such as the T105 are the cheapest way to get watt-hours from a battery. Except for free scrap batteries that is.
Some day soon Lithium Iron Phosphate batteries will catch up. They charge faster and are much lighter, so for applications such as laptops and some electric cars they make sense now.

One final thing: After the test, charge that battery up and keep it charged! Sitting around in the discharged state causes sulfation and loss of capacity in lead-acid batteries.