Most batteries that fail the performance test are fully charged when they come into the shop, dipping into the cells to check the specific gravity of the electrolyte is an unnecessary hazard.
We were using hand-held conductance testers in the 1990’s, they can be difficult to get a “failed” reading to get a warranty code for replacement.
For the last 15 years we have been using more involved test equipment for warranty requirements from the vehicle manufacture and Interstate Batteries. Many Manufactures require dealers to use the Midtronics GR8 to test batteries. This machine uses a combination of charge, load and conductance tests to check the battery.
The Midtronics GR8 is being phased out, replaced by more modern equipment, the Midtronics DCA-8000.