[old man neri]

Around here you can take your battery to any FLAPS and get a free load test. Should be a good guess as to whether or not is will last the winter. I think you can also buy a tester for cheap, (it's nothing more than a heating element and a gauge)

Also, another good indication is to look on the battery and find the date of manufacture. 5-7 years and your batter is probably done.

Lastly, the your best bet is to carry around either jumper cables or a boost pack and when/if the time comes just jump it and drive to the FLAPS and get a new battery.

Depending on how cold it gets where you are the purchase of a battery warmer or a block heater may not be a bad idea.