Most newer batteries are maintenance-free. These type of batteries cannot be visually checked. Older batteries have removable caps that expose the cells inside the battery. If you have an older battery it probably needed to be replaced 10 years ago.
While most batteries are maintenance-free, the battery posts are not. The posts or terminals are located on the top or side of the battery. Inspect them to make sure there isn’t any corrosion around the post.
WITH THE ENGINE OFF, wiggle the two cables attached to the battery. Make sure they’re not loose, cracked, or frayed. Even the slightest gap between the cable end and the battery terminal can prevent your car from starting and/or charging.