Qualitative analysis of production logs with an emphasis on temperature logging. Make your decisions based on knowledge, not assumptions.
Most production logs run today are on wells with poor performance due to water loading, wellbore damage, communication behind the pipe, leaks, and other factors. It has been my experience that a large percentage of these wells do not have production at, or above, the minimum amount required to obtain a "high confidence", quantitative analysis.
Properly run tests can lead to the identification of many problems that can affect production rates through qualitative analysis. While you probably cannot determine exact values of flow, you maximize the opportunity to discover the problem and obtain the information needed to make an economic assessment of possible solutions.