Given that most cars do not come with voltmeters and batteries have a habit of dying without warning, how do I ensure I am not stuck with a flat battery?
The best way to check a battery's condition is to measure the voltage across the terminals while cranking the engine. Anything less than 12 volts indicates that the battery is nearing the end of its life.
At rest, the battery voltage should read between 13 and 14 volts, even though the battery is specified as a 12-volt DC power supply.
As you correctly pointed out, most cars do not feature a voltmeter, so the only means to check the condition of the battery is to have the voltage measured by a battery shop.
Almost all of them are equipped with a simple instrument to measure voltage. While you are at it, ensure that your car battery's voltage is no less than 13.5 volts during idling. With higher revs, the battery voltage should rise to at least 14.4 volts. This will tell you if the alternator is performing well.
Besides this, the best indication of a dying battery is, of course, slow cranking. That means when you turn the ignition key to start, the engine starts up slower than usual.
While this could also suggest a weak starter motor or starter motor wiring, observe if your interior lights or headlights get brighter when you rev the engine beyond idle speed.
A battery which is nearing its end is unable to supply enough voltage to maintain illumination brightness at idle speed.