The correct answer to this open question is the following.
Although the question provides no context or references, we can say that if the top-level management team has accepted your recommendation their effectiveness can be evaluated three months after implementation in the following way.
The recommendation needs to establish some goals that have to be accomplished in the short, medium, and long-range. After the first three months, you establish your exéctations and you should have included your KPIs or Key Performance Indicators in order to do the proper evaluation and knowing if the recommendations were valid or attainable. Lack of goals or KPIs to evaluate the recommendation would end up complicating the evaluation process.