The most common sign of a bad car battery is a car battery that is slow to crank the engine, clicks the starter, loses it's charge often or doesn't charge back up.
If you have to jump your vehicles battery and you haven't left anything on in the car to drain the car battery such as the lights, accessories etc then it's time for a new battery.
Batteries have a lifespan of between 4 years to 8 years but if the cars battery is 5 years or older and you're noticing the car cranking more slowly then it is likely time to replace the battery before you get stranded or need a jump start.
I replace my batteries in my car and truck after 5 years regardless if they are still working and then I keep the old one in case I need it or someone else needs it.
I've given an older battery to someone who could not afford a new one at the time and my battery was older but it was still good.