You can get a cigarette lighter adapted volt meter that tells if your system is charging. I have used one on an old boat I had, which had some charging problems due to the batteries being fairly old.
Then again those batteries are like 3-4 feet long by probably 16 inches wide.
Now thats a big battery.
If you need to replace the battery, you might want to look into one of those new spiral cell batteries, like Optima. I havent used one in my car as it had a new battery in it when I bought it, but if I need to replace the battery, I'll probably go with one of those. I think you can get ones with well over 1000 CCA. In fact they have one thats 1050 CCA.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.