A hydrometer will tell you State of Charge, but not State of Health
, except, in very general terms comparing precisely calibrated readings over time compared to benchmarks established when near new, looking at changes over long time periods.
A 20-hour load test is the only reliable way to determine residual capacity, at least without lab gear
and a bank should be replaced when that falls to 70-75% of rating when new, industry standard is 80%.
Most people just keep using until performance is noticeably impaired, take twice as long to get back to 100% Full.
But that leaves you open to "sudden unexpected failure", best avoided.
Others just replace on a schedule, say every five years.
If you want details on doing a 20-hour load test, Maine
Sail's site has that, in one of the BM articles.