Tolga,
A hydrometer test to each cell will quickly point out which cell is no
good.
Also a battery load tester hooked to the battery and loaded up will tell
you what condition the battery is really in.
Take that same load tester, keep the gound wire connected to the
battery, and wipe the positive cable over the top surface of the
battery.
This will tell you which ( if any ) cells are shorted, by watching the
meter fluctuate when you are close to the shorted or bad cell area.

RC