The only accurate way to detect a weak battery is with professional equipment. But do-it-yourselfers can do routine maintenance. Scrape away corrosion from posts and cable connections; clean all surfaces; re-tighten all connections. If battery caps are removable, check fluid level monthly.
A word of caution:
Be sure to avoid contact with corrosive deposits and battery acid. Wear eye protection and rubber gloves. Note too that removal of cables can cause damage or loss of data/codes on some newer vehicles so refer to your manual for instruction