As with so many things, "20/20 hindsight".
I try to "eyeball" stuff often. At least weekly, usually daily (unless I'm out of town). This doesn't mean a full maintenance - it just means being "aware".
Anything smell odd? CHECK IT!
Anything look unusual? CHECK IT!
Voltages seem unusually low or high? CHECK WHY!
I also periodically (at least weekly) just quickly go past the battery bank checking the temperature of each cell. Only takes a second or less per cell (I usually do two at a time). Then run back along the bank checking the temperatures of the interconencts themselves.
My trojans are all edge-on-edge, so I check the case temperatures, and also give each one a gentle "thump" with the end of my fist. I once found a cell going bad because it was consistently gassing much more than any of the others by this technique - give each cell a light thump and sometimes they gurgle as settled gas is knocked loose.
This goes for just about anything around the place. Motorcars, generators, turbines, pumps. Just "be aware". When things change, investigate WHY something is different. If the water level in the dam or your tank suddenly changes, check WHY. If your hot water is suddenly hotter (or colder) than usual, CHECK WHY. If your icecream is soft, or your milk goes sour after only 3 days - CHECK WHY!
I know another thread goes on about the danger of having your batteries inside and how they need to be locked way in a bombproof shelter at the far end of the garden, but honestly - how often do you check them? "Out of sight, out of mind" is not just a saying - its a reality.
YMMV, just my opinion, etc.