Chemical control can be basic or advanced according to the equipment available as well as the skill and knowledge of the operator.
With these measurements it is possible to tell whether a solution is in or out tolerance, but not what is incorrect.
The formula for each solution will give a figure for pH and SG and a tolerance.
The SG reading will tell you if the solution has the correct quantity of chemicals dissolved. If one chemical were left out of the solution a low SG reading would be obtained.
If comprehensive analytical equipment is available then it is possible to determine a complete analysis of the solution using methods documented by the stock manufacturer. Any incorrect solutions will be detected.
As with all types of measurements there will be differences in measurements for pH and SG due to statistical variations and such things as the amount of water absorbed by the chemicals before weighing affecting the concentration of the solution.
Again trends are looked for or gross deviations.
A recheck of any gross deviation should be made to eliminate operator error.