Hydrometers only work on older style batteries which are not very common these days. They won't do you much good on today's sealed batteries.
VOM set on the 20 DC scale is a more modern approach - 12.6 volts after sufficient time for charge bleed off. With that said, I've seen a battery read 12.62 one day and go stone cold dead the next.
Bottom line - you never know when a battery is going to go no matter what method is used to test it.