How to measure a functional capacity of employees?

Previously I explained the situation when in the situation of laid off people, tasks are divided between rest of the people. However, in institutions and companies where salaries are decreased but at the same time due to structural reforms, there is a need to delegate more tasks to one employee, people are complaining that there is an overload of work.
Previously I described other problem, but now needs to take in consideration a tendency when institutions and organizations during growth of economics hired too many employees who were not fully employed.
Considering nowadays situation when many companies and organization should allocate recourses, work more effectively and cut expenses at the same time employees should work more to receive the same salary, also in government institutions people will obtain more and differed tasks.
But the question I am wondering about is how to understand and measure the level of the capacity for one person doing a work in a good quality?
One measure of the quality and capacity can be the level of satisfaction for the person doing the job. It can be measured with different personnel tests. Second a personnel specialist can interview employee to evaluate his motivation and strengths. Each organization should have a clear vision of the people satisfaction of doing work.