The only reliable method of determining the state of charge of a lead-acid battery is by measuring the specific gravity of the electrolyte. The SG for a fully charged cell should be around 1.26. The specific gravity rises in an almost linear fashion from about 1.1 for a fully discharged cell. Battery hydrometers are fairly cheap and readily available.
Sadly, with the advent of sealed, gel and maintenance free batteries this method isn't always an option.
The terminal voltage isn't a reliable indicator because of the one of the most useful characteristics of the lead-acid battery; its ability to maintain a near constant voltage over the bulk of the discharge period.
During discharge, the terminal voltage remains almost constant at about 2.0 volts per cell, dropping to 1.8 volts at the very end of the discharge cycle. While it's on charge, the terminal voltage will remain at about 2.2 volts per cell, rising to 2.4 volts at the very end of the charge.
The only other method of "guesstimating" the charge state is to monitor the charging current during the recharging period. The actual charging current will depend on the capability of the charger, but generally the current will be very high when first connected falling to a steady level for a number of hours. When a significant drop in the charge current is noted, the charge cycle is nearing or reached the full charge.
Lead-acid batteries are generally more suitable for slow charging over 8-12 hours rather than fast charging, although that will depend largely upon the design of the battery.
A "drop test" whilst not reliably indicating the state of charge of the battery, it will often show a battery which is nearing the end of its useful life.